Hot cold confusing?

Basically there is this girl who i think i am interested in, now she told me on FB that she found me attractive, but i have incredibly low self confidence, when it comes to someone paying me a compliment or is attracted to me, i instantly think their mocking me, but she seemed sincere.

Now i met with her the other day and she was incredibly flirty with my mate who she had also slept with. Now this does not bother me really, what does bother me is that i told her i was surprised she fancied me and that i thought she was interested in said mate because i had seen previously the two of them touching and flirting right in front of me. She assured me they were just mates, and she had no real attraction to him. And thats cool as they are flatmates, and everyone needs a casual releasing now and again.

Now is this a common tactic between girls, cos generally i am straight up, and have no time for wasting energy on someone who maybe playing.

The weird thing is, she openly (through IM) asked me if i found her attractive, when i said i did she reciprocated.

I approach things a bit different to the average male, i really have to get to know a girl before i full on fancie them, as i can be attracted to what i think is a 10/10 but then 15 mins of talking to them they become a 3/10.

HOW SHOULD I APPROACH THIS MATTER ?