What Causes Delayed Period Despite Having Negative UPT?

I am 12 days late for my period. But have had 4 negative home pregnancy tests. I just came off tri-sprintec about a month ago. I am not trying to concieve. I am just wondering how llong it should take for my period to return because I have been having what seems to be symptoms of my regular period for about the past week or so. I can t get an OB GYN appointment for another 2 weeks. And I m desperately searching for some sort of explanation because I ve been on birth control religiously since my daughter was born 14 months ago.