Girlfriend's trust in my virginity.?

It has been bugging me so much since yesterday. I've been going out with this girl for about 4 months now, and we started out as being best friends. She randomly asks "Are you a virgin". And of course I tell her the truth, which is "Yes, I am a virgin". She responds with an "Uh huh...sure". That bobbled many thoughts. So I then asked why doesn't she believe me, and she says "how about your ex? you went out with her for like 5 months, and I'm pretty sure you did something then. Then I tell her Me and my ex did nothing besides kissing. My ex was an uncaring girl, and she knows that. So then I asked if she trusts me, and she responds by saying "Yes of course I do, Just not about this." So I don't know what to do, It's making me very sad, and I don't know how to convince her other wise.