I wouldn't call it the start to another cycle. I would think ovulation bleeding, but after looking at your chart I'd say that Wednesday would have been the only possible day you could have ovulated so far, so the bleeding seems early for ovulation bleeding. It's possible that it's some break through bleeding happening from your body adjusting to not having the Mirena anymore. Your fertile CM could be a sign that you did in fact ovulate recently!
Are you temping at the same time each morning while still lying down? The temp can fluctuate up to 0.1 degree per 30 minutes, with a lot of factors that can also influence it. If you're getting up to pee in the middle of the night that could be messing with your temp too. I'm pretty stringent about my temping but I'm going to try vaginal temping next cycle to see if it tells me anything different about my cycle. It's supposed to be more accurate. Hopefully I'll get my BFP and won't have to worry about it all though!