Good day! My first day of my last period was Aug.01-08.. I have a contact with my boyfriend last Aug.20, we used a withdrawal method.. Then, Aug.26 I have my period again, that was lasts until Aug.31 but on the first 2 days that was Aug.26-27 I discharged brown blood then on Aug.28-29 I discharged red blood but that is not flowing unlike regular period and my napkin is not full it's like a light bleeding and on Aug.30-31 I only spot but it is not a bleeding type. 3weeks after conception, I tried to have an HCG test but it was negative! I don't have any signs but sometimes I get a migraine, mild lower back ache, mild bleeding gums when I brush my teeth and a mild tonsilitis. Last week until now, I get some abdominal cramps but still my period is not arrive.. Am I possibly pregnant? Or hormonal imbalance only? My cycle is between 26-28 days.. I hope you can help me, thanks a lot!

Pregnancy tests don't work 3 weeks after conception.

You will have to wait to one week after the next missed period ( if you miss the next one)
You do understand withdrawal is not really effective and has the highest failure rate of any method