Google sleep paralysis. It sounds like you may be experiencing it. There's a group here on EP for it too. Many of the stories are very much like what you've described.
U GOT SUFFACATED BY A HOT CHIK? DAMN, KINKY. U LUCKY BASTERD!!
it could be how someone died and they're asking for help. most of the time ghost aren't trying to hurt u.. they just want to get ur attention the best way they know how so u can help them rest in peace.. maybe she needs ur help crossing over.. she might have got stuck here because there was some unfinished biz.. they get stuck on earth when thier death was fast or unexpected. she just wants to tell u how she died and maybe who killed her.. if u can help her then she could rest in peace.
Is she holding the pillow that she had over your face just before you woke up?