Why do i feel like a creeper around her?

I'm 19 and she's 16 and we met in our apartment complex over the summer when i was 18 and there were friends my age and older and we'd all hang out and stuff. Since then, all the friends i had that were my age moved away either into houses or other apartments or what have you. I turned 19 earlier this month and she's still 16! she's really cool and nice and stuff and i don't like her or anything but it feels WEIRD! and she's really short so it makes her look even younger than she is! should i stop hanging out with her because ever since i turned 19, i've been avoiding her and she's friends with my sister so that makes it even harder to do because they keep badgering me to hang with them and it makes me feel like a pedophile or something! what do i do?!