possible pregnancy?

Is it possible to be 17-18 weeks
-Have like 12 negative home pregnancy tests (I used different brands and took them at all different times of the day including fmu) 
-Have periods each month (on the pill) bleeding for 5 days on my placebo pills , 2 days heavy- medium and 3 days medium to light. I have had  spotting each cycle 3 days before I take my placebo pills. But I have been on antibiotics. 
PLEASE HELP. I have had period cramping everyday for over 4 months now. 
Thanks