Bleeding during sex and no period

My husband and I have been trying to get pregnant for two months. Last month I was tracking my ovalutation and we had sex for 3 out of the 6 days. Two weeks ago I was feeling bloated, gassy and tired more than usual. I was supposed to get a period tomorrow. Last night we had sex and I started to bleed, as if I started my full on period. Woke up this morning and nothing. I have a little, very little brown spotting, almost nothing but the cramps and stomach ache is there. Took a pregnancy test and it was negative. What is happening. I always had regular period for 3-4 days, nothibg what I'm experiencing now. What should I do? Anyone experienced anything similar ? Can I be pregnant? Should i see a doctor ? Should I retake a pregnancy test ? If so, how long should I wait ?