Girls!!! I need advice!! Not sure what to do in this situation.?

I've been hanging out with his girl I went out with 2 years ago for maybe a month or so. Recently, me and a friend(my best friend, also her friend) have been hanging out again out of nowhere. Whenever we hang out, it's always the 3 of us and that's just the way it is. Then 2 days ago, she sent me a message asking what I was doing today, I said nothing and before you know it, it's just me and her going to a see a scary movie tonight. The last time I hung out with her, she seemed a little flirty towards me, but I didn't really realize that until after I had left cause I'm a dumbass.

So we went and saw the movie last night, having a really good time. Wasn't stressing at all, then she really started leaning into me during the movie but I couldn't tell what she meant by it. I would have easily put my arm around her, it's just that if I did it and it wasn't what I thought, it could easily worsen our friendship. So after the movie, she wanted to hit up taco bell, so we went in, got us some grub and that's where we had a really good time. I had her laughing hysterically a couple times but I still couldn't read her 100%. My only question to you guys is....what should I do? I'm really starting to re-like her again as more then friends but I'm just not sure of the best way to approach it. I just want to go the safest/smoothest route where I can clear things up with her but have her not really catch on 100% at the same time.