There are so many things that can tamper with your cycle it is hard to ever really know. I would test again...most early detection tests would definitley work accurately by now. If you test too early it will not be accurate. When I was pregnant with my 2nd I had 1 negative test 1-2 days after the day I was supposed to get my period and then I tested 3 days later and it was positive. Funny you mention antibiotics though...I just finished taking 10 days worth and my period is late by 5 days but I am about 98% sure I am not pregnant. If you want to be I hope the best for you and if you are not...you can try try again!