I find it hard to tell what flirting is at times so think people's definitions of it might be all over the place. It sometimes feels like "was I just flirted with? naaaa" and forgets about it...

If you felt you weren't "flirting" then you weren't. I used to get told I was flirting with people all the time during reminiscing-fests and it blew my mind as I wasn't thinking that way.
So unless you can recount the dialogue it would be hard for anyone to put it through a "flirt filter" of sorts, tone also matters.

That depends on how the other person(s) perceived your actions, and of course they can be wrong, which appears to be so in this case.

It's tough when you have been out of the dating pool for a long time. This is a very different era and very different way of things. Flirting to me is expressing appreciation for someone's attractiveness and personality on a male/female (talking hetero here) level to express interest without creating an awkward situation where real expectations are involved.