Me and my girlfriend teen girlfriend were fooling around.I asked her to grind me in the nude.so we took our clothes off and started rubbing each other.her hymen is intact and there was no penetration.however there was a drop of precum in the tip and I don't clearly remember if my tip touched her vagina or not.this contact didn't even last for more than 10 seconds.but she sat on me.i did not ejaculate near her or later.her periods ended in the 13th.and this happened on the 24th night.i am wondering if 

a) this happened on her fertile day?

B) if she will get pregnant

C)what can I do to stop her from conceiving.