Please help! Things are going kind of fast and my feelings can't catch up?

I started talking to a girl on Facebook like 2 days ago and we just clicked immediately, and now we're good friends. She messaged me to get on this morning and I got on and she said "yay!" and whenever she says brb and comes back she'll ask if I missed her, and then she puts a bunch of "jk"s behind it, but she does it every time. Today I asked her for her #, and honestly I don't have any idea why I did so. After that I got really confused and I told her I had to go so I could think. We compliment each other all the time and we just seem to get each other. She's in my grade and I know her, but we only became friends 2 days ago. A few minutes ago I went back on Facebook and saw that she was on and clicked off immediately. I'm so confused... Why are things going so fast between us? I wouldn't mind being more than friends eventually... But it's just too fast. I think I'm scared of anything happening. Does she have have an interest in me or anything? I don't know if she does or if that's just her... I just don't know... Any input?