I Am 27 Yrs Old. Last Month My Periods Were

i am 27 yrs old. last month my periods were from 4th-9th dec 2015, had sex on 17th of dec. this month i didnt get my periods, waited for week but still didnt get periods. today morning i took pregakem test early in the morning, results were negative. gained weight of 5 kgs. am confused whether am pregnant or nt ? periods skipped and test came negative...confused. please reply