I don't know whether i'm pregnant or not?

I know i might seem like a freak but i really want to know what should i do, I had an unprotected sex on the 6th day of my period 2months ago and i got my period on time 20 days later same as previous ones heavy with crams dark with clots and lasts 6 days, this month i hadn't had sex ,but i'm still waiting for my period ,i'm now 3 days late and did a pregnancy test that was negative, i have no pregnancy signs ,i gained 2kgs( i know it is not specific but anyways) do you have any suggestions?please let me know and tnx