Feeding related. Of not breastfeeding!

Hi, I have recently delivered baby 2 months back.. Initially he used to breast feed at lot but from only one breast. I used to supplement it with some formula, But since last 15 days, he is totally relying on formula... Since the day he has stopped breast feed, my one breast through which he used to feed is a way more bulkier and bigger , even that side has a lot of milk production.. Whereas the other side is quite lighter and smaller and has less milk.. I am trying to express my milk through breast pump regularly. Even try to feed him the expressed milk but he don't even take that. I am really worried about being difference in size of breast, as the breasts r painful too... M worried if it can be cancer too or just the side effects of not breastfeeding