The Past Two Months My Period Has Been Short (2-3

The past two months my period has been short (2-3 days) and dark brown when it is usually bright red and lasts 4-5 days. In between cycles, I have also gotten a few days of spotting of the same type of dark brown blood and very bad back cramps that cause me extreme pain. I noticed the spotting in between periods happened right after drinking a glass of wine or mimosa or two and the last time I drank a mimosa I had spotting for a whole week, no where near my cycle start date. I tend to have regular menstrual cycles as I am on the pill but these last two months have been horrible as I have described above. I know I have read on here already that alcohol doesn't cause vaginal bleeding but I was still curious as it has happened more than once, especially after light alcohol consumption, and wanted to know if there was anything else it could be.