General Question?

My periods have always been regular, but now after taking this medication for over a month now, im only spotting, my periods are normally very heavy and last 8 days or more, but this month is only spotting, nothing at all on my Pad. for 3 days now, i took a home PG test to just see and that was negative. I dont know whats going on with my body. Im also taking Doxycycl (Antibotic) and Hydrochorot for blood pressure, i dont think these would cause this, but someone else could help me understand whats going on with me. Thanks