Hi, A Previous Sexual Partner Just Contacted Me And Said

Hi, a previous sexual partner just contacted me and said he think he might have an STI. We had sex 2 weeks ago and he thinks he got it from me. I had been tested negative the week prior to that so I don't see how that's possible. I'm getting tested again this week to be safe but I was wondering if you could give me some insight to what it could be if I tested positive and it's only been two weeks