Have you become attached to her very fast? From what I've read from these forums, people tend to move fast with their rebounds. Perhaps that's what she's worried about? Still, I find it weird to jump to such conclusions considering you've not talked about your previous relationships.
Talk to her about this. Ask her what gave her the impression. Try to assure her that this is not the case and keep dating her like you would.
I would suggest you to try not getting too involved with her emotionally until she's over this impression. Not knowing the girl, she might be pulling some weird game on you, who knows.
You can't buy love, but you pay heavily for it.