Scared

So my bf and I had unprotected sex Sunday, which little did I know was the first day of my fertile window, he did not finish inside me (can you get pregnant from Precum?) but it is now Wednesday and I'm getting concerned I've been stressing about it constantly and don't know if this is related but I've had some weird stomach cramping and almost feeling a little weird and nauseous (this could be related to medication I was just on when I had mono). I'm just freaking out and not sure if I should be this worried