HELP PLEASE

So I had unprotected sex November 11 and like two days later I started to feel weird like when I'm going to start my period. But I was scared that I might be pregnant so I took a test and it was negative. A week after I had sex I got my period which was November 18. And since that day I haven't had my period so it's going to be 3 month on February 18. Today all day I've been feeling nauseous and my stomach feels weird like if I was gonna start my period but nothing... What could it be? I really hope it's nothing serious tho!