How To Calculate Menstruation Cycle Length?

hi im 21 and not on birth control. last month i got my normal period from the 16th till the 20th had unprotected sex over the next 3-4 days and then i got another period on the 30th that lasted 4 days but was different. now i dont know when to expect my period this month do i count from first or second period or am i pregnant? i took a test yesterday and it was negative but could it be too early to test?