Flirting is like comedy and taste in music. You really can't give it ... a textbook definition on what it is for each person and how other people (or more importantly how other individuals will react).
I am also talking to another, rather shy girl who would occasionally make comments like "Uhh I sunburned My ass" or while she was laying at the beach "I might not be wearing anything right now just FYI". Is that flirting? How do I react? What do I say? I once said "pictures or it didn't happen" as a joke and she just started another Topic. Did I f*** up?
Given very little context other than this person is shy? You kind of fracked up here. We can't tell if she's prudish or sensitive or simply doesn't like you in that way.
The only advice is too not be too creepy but don't fake flirting either. Other people can see right through disingenuous flirting (flirting for attention grabbing sake as opposed to flirting because you have genuine affection or attraction towards the other).
You shouldn't ask others what flirting is. You kind of need to learn what your own style is. Everything in the pursuit of romance is trial and error. It's trying to find a balance between what makes you confident and comfortable while not creeping the ever loving crap out of the other person.