What could be causing this?

I'm 26 and taking prometrium CD 17-25 to kickstart AF. Today is CD 27 for me and I had a positive OPK on CD 13 My last cycle was 37 days and the previous was 34, so I don't expect a period for a few more days when my progesterone levels drop from not taking the medicine anymore. On CD 24, I had extremely light, pastel pink spotting when I wiped. I thought maybe it was implantation bleeding, which was exciting because we've been very hopeful this month. I had no more spotting until the night of CD 25. At work that night, I had to lift a pretty heavy box, started getting cramps, and started bleeding more consistently and it hasn't fully stopped yet. The color and consistency are much different than my normal, heavy, periods. I usually change a tampon every hour to two hours and I haven't filled even half a tampon so far. No clots either, which is extremely strange for me. I'm also no longer cramping, no backaches, no breast tenderness. I'm not having anything close to what I normally experience. I called my doctor and was told not to worry, but this is worrisome to me. Has anyone experienced this or something similar? If so, what was your outcome?