I was seeing this girl, then there was an incident with her ex-boyfriend what should I do?

I have been seeing this girl for a few weeks and we both realized really quickly how much we have in common and how much we enjoy spending time together. She sat me down and told me seriously that she had just gotten out of a long relationship a few months earlier, and that she just wanted to take things slow for now. Totally fine by me, we keep dating, keep learning more about each other and so on. So one night after having dinner with her when we said good-bye, I told her as a step forward that my kisses are only for her. The next day I went to surprise her and visit her at work to find a big hickey on her neck. Well I asked her about it, and she told me the whole story of how her ex had come up the night before to bring her money her owed her and they hooked up, no . She was really sorry, and told me she wanted to move on, to be with me, that she wants to continue to develop our relationship. I just don't know if I can trust her now? Please help.