Metformin & period?!? 🤔HELP!

Hello!

I've been back on Metformin for awhile now, but I still have not got my period. Then, this past Thurs (8/24), when I went to the bathroom, I wiped and there was brown tissue/spotting with mucus! I even had the lower back discomfort as if I were going to start my period... BUT nothing more came and cramping stopped! I thought, well, maybe this was implantation?! But today, four days later, I had a negative test.

So, I guess my question is, has anyone else experienced this? Should I mark it down as a period? Or could it have been I'm pregnant and tested early? All periods are super irregular, so there's no way to know when I would have a fertile window... please help!