How Can I Test Positive For Pregnancy With HCG After Having Normal 3-day Periods For Two Successive Months Post Intercourse?

I had sex on 25th January, I had my period (menses) the following week which was February and it lasted for 3days. And also had my Period (menses) in March which also lasted for about 3days. But I've tested positive for pregnancy with the HCG rdt. I'm confused. Can there be any explanation please ?