My blood hcg is at 1 and it's been anywhere between 3weeks to 3 days since I've had sex

I know it says anything under 5 is negative but I've been reading a lot and a lot of different sources say up to like 3 weeks you will be under 5 to 10 hcg should I go back and test in a few weeks? I dont under stand why I have hcg iny blood but not pregnant can someone explain it to me please? I feel really sick and my doctor wont give me anything for it till my results are confirmed