This is pretty common. It’s because of the flush of hormones being released into your bloodstream. As you lose weight, the oestrogen that was stored in your fat is released which causes the difficult periods - heavy, bad pmt, irregular, etc.
Being menopausal, I had the reverse situation. All the oestrogen stopped my menopausal symptoms. Unfortunately, now I’m in maintenence & all the stored oestrogen is gone, the hot flushes are back.