There's no way to answer that with the information that you have. Girls have a membrane in their vagina called a hymen. This membrane tears (usually) at first intercourse and that is what produces the small amount of blood and discomfort. The amount of blood naturally varies depending on the person, and in the modern world many girls have stretched and/or partially torn their hymens with tampons, masturbation with objects, and even athletics. Therefore the amount of blood isn't a reliable indicator.
To make a long story short, there's no way to know with the information you have.
Now, I'm going to give you the real problem: If you can't trust her word, why are you with her to start with? If you can't trust someone's word, then you don't have a relationship.