No period on tri sprintec?

A little background information first. I've been on tri sprintec for about 3 months, went on it 2 weeks after a miscarriage in December. So this would be my third month I guess. Its the 4th day of the sugar pill week and my period has not started. I took two pregnancy tests, 1 yesterday at 1:30pm and another this morning at 6am and both tests came back negative. What could be causing my period to not start?

I'm not ready emotionally to be pregnant again(or health wise) which is why I'm on the pill. What else could it be?