What happened?

So my girlfriend and I were alone at her house and we were playing around a little bit but when we got up to her room and things were about to happen she stopped me. She wasn't on her period or anything and she wouldn't give me a reason for why we stopped. She just says "don't worry about it. It wasn't you". But now all I can do is worry about because I want to know if I did something wrong or if I can just understand what happened.