CAN HPTS GIVE FALSE NEGATIVES?

Is there any reason why a home pregnancy test wouldn't show pregnancy? 
I have been taking pregnancy tests since I last had sex in febuary, I would be 19 weeks exactly now from when I last had sex to the date. Because I'm on the pill I don't even know I would or could have ovulated. 
I have taken 12 pregnancy tests since I was possibly 3 weeks. THEY HAVE ALL BEEN NEGATIVE, at the start I never tested with first morning urine so it wasn't till I was POSSIBLY ABOUT 12 weeks that I tested with morning urine. They have ALL been negative. 
Should I believe these tests? 
Yes I have been getting my period, but they have been coming a couple of days early with 3 days spotting, two days medium to heavy and 3 days of medium to light. I'm due for my period in the next few days and I have sore boobs so I really hope my period comes this month considering I could be 4 and half months I would think my periods would have stopped.