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Hello, I'm Dr Manos MD, board certified obgyn. My answer to you is to have someone do a transvaginal ultrasound to see what's going on now. It's unlikely that hyperplasia and a polyp could survive through the suppression of pregnancy and breast feeding. I'd be interested to find out too.
I was thinking about the mystery mass and maybe you have a bicornuate uterus. Or a variant of that where the top of the uterus looks like a heart as the top wall puckers in and there is a septum inside but not totally separating the inside space but partially. The mass is actually the incomplete septum.