Ha!
I had the exact same experience, with my mother and everything!
30+ years acive in the church, but severely doubting my faith for the last 10 years. In all that time, I never once engaged in any Mormon-defined “sinful behavior”.
... but naturally, as soon as I officially became public about my choice to leave church, my mother’s first inclination was that it was because of some secret alcohol/drug/sex addiction or something.
After I did my best to clear that notion up, her next assumption was that I wanted to leave the church BECAUSE I I wanted to get into alcohol/drugs/sex.

... it was such a sad revelation to see that even my own loving mother instantly assumed the worst about me, rather than hearing that my choice to leave was based on a lack of faith in the doctrines being taught to me. To me, this completely encapsulates one of the fundamental problems of Mormon culture: My mother, like many other active Mormons, has been indoctrinated to the point where she can’t just see the forest for the trees. It’s so sad.

Now that I’ve been out of the church for a couple years, I’ve found that coffee is amazing, and scotch is unbelievably tasty!
Isn’t life outside the prison of the LDS faith absolutely wonderful?!

They will use that one against you since we all "sin" by their definition, even if it has nothing to do with why you are doubting their beliefs/scripture.

I was raised in a fundamentalist church, I had a strong desire to do the right things, however, I also had a strong desire to know with certainty what was right! Therefore I questioned everything. It didn't take long for me to become unpopular at any religious gatherings! I was generally searching for the truth and always coming up short. However, I found Science and the scientific method fairly fruitful and reliable given enough time to sort out the garbage. I still study religions of the world along with world government, philosophy, history, and science, but I hold all things to the torch of science for the closest thing to truth we can achieve! So back to the original query, if questioning the beliefs of a religion is a sin, I then was and am in a constant sinful state!

I grew up Mormon too, and I had no problem following the rules. I remember when I left someone saying: but I thought your testimony was so strong!
I’m standing there like, well, then you don’t know me very well. Have you been listening to what I have been saying? I’ve been talking about how I disapprove of the sexism etc. in the church.
Just because I studied and knew my shit and had no trouble following the rules did not mean that I was on board with everything.

Oh, definitely. However, it was probably my attempt to reconcile my former faith to my bisexuality that put the nail in the coffin.

Well I was never an ardent believer. You can say I was a rationalist who then became an atheist. So no major repercussions

I really never felt sinful. I have always followed a high moral compass and while I am not perfect I strayed a few times but did so without guilt. No god to please, no redemption necessary. Live a good life. Help others get through it and lean on a friend when the going gets rough. God didn't do it for you, to you or through you. Own it yourself take the credit, admit the blame carry your own water and help others when they can't carry theirs.

For those of you who were formerly religious, were you engaging in behavior that your religion deemed "sinful" when you started questioning your beliefs?

I was mid-teen, age of reasoning for myself. I was for all matters, a good kid. Nothing sinful or at least nothing that I deemed was.

How about after you were fully out as a non-believer.

I was still me. I did not act out more because I choose to stop believing in the fairy tales. Although those fairy tales were embedded deeper than I thought. 20 some years later someone was singing songs I had not heard since .. well, I was a young teen. Flooded memories and resentment and such.

Hi Qryzti,oh yes I indulged in very sinful deeds according to the bible lusting after women with many many one night stands and loving it.of cause this was a long time ago when I was in my 20's.Still love sex but at 76 it is more of a memory lol.

I was a true believer as a child, upright and faithful to God. Then God burned down our church and I found doubt.

As an adult my doubt gradually turned to disbelief and then to Atheism, so there was not an abrupt transition.

My first love was a christian woman, and she challenged me to find my belief. Unfortunately for our relationship, it took me the opposite direction. There was still a lot of sinning going on though, my bad. I felt guilty about the sex, but not because of my own feelings, but because I was tempting her to betray her own beliefs.

I still try not to lie, steal, or hurt people.

I was very young when I started to have doubts and hadn't had much opportunity to sin. I started out my sinning life with a big one though- I tried to summon Satan as part of an experiment to see if God was real!

As a catholic I lied under questioning in the confessionary. I received the "sanctum sacrimony" while under sin. I am still here over 55 years later.