Would need more info but I imagine you probably get too emotionally involved too soon. What do you say the morning after?
Are you saying stuff like:
"So, what are we? I just want to know where I stand."
"So, are we going out now?"
"When am I going to see you again??"
Are you texting/calling her a lot? Have you stated you are looking for a girlfriend?
If so, play it much cooler. If she suggests a relationship then fine, but I think you are probably being too needy after the sex. I sincerely hope it's not because you're a crap shag. Say something like"hey I really enjoyed last night. You've got my number. Call me when you want to see my darker side...

"
Be prepared to lose the girl to get the girl.