i need some help please bout whos baby?

This is a horrid thing to go through and i hope maybe someone can help ease my mind, so my last menstral period was 29th of july - 4th of august ( my periods are irreguler ) on the 4th of august i was at a work due and my drink was spiked and i woke up next to a work mate i was scared i new what had happend i could feel it he told me not to wory he had pulld out before he cumed August the 8th i took the morning after pill the 1 that is efective up to 5 days later august the 10th i slept with my partner. august the 13th i started to bleed very light only blood when i wiped, stoped on the 17th and i slept with my partner everweek after that on the 19th of september i took a pregnancy test and it come back a very faint almost not there i had to hold it in every angle to see a 2nd line 21st of september the doctors took a pregnancy test it came back negitive so they sent me for a blood test and my hcg level came back as 27 24th of september i went for a scan an internal one and they said there was no sign of pregnancy 9th of october i went for a scan and they told me ges age was 6 weeks. Im so scared i dont want to tell my partner of what happend because he may want to hurt my work mate i just never speak of it and the work mate has left, please who is the father? Is the only way getting a dna. Im scared im 25 and have 2 kids already if my partner was not the dad i think he will abandon all of us! Please anyone who knows what there talking about help me out, what was the bleeding on the 13th was that my period?