rejecting one side and the other one is gettibg sore

[ObsidianBlackbirdMcNight]

from overuse i think. why? ouch

Could you have a blocked duct so the milk is not coming through hence the rejection? Have you tried a different position on the rejected side, the rugby ball position for exampe? Perhaps DC cannot get a good enough latch?

Lots of nipple cream might help the soreness?

Do you have a lump at all. Very rarely a baby can reject one of the breasts because there is a lump there, whether it's a blocked duct or something more serious. Have a good feel round. What I would suggest is that you express as much milk as you can from the breast that is not getting used and then check for lumps. This will help identify any blocked ducts or other abnormalities (don't worry, the vast majority of lumps in boobs when breastfeeding are completely normal).

Next try expressing a little milk from the unpopular boob (obviously wait a bit for it to refill first) so the milk letdown is stimulated, and then attempt to latch the baby on. Try and do this when your baby is sleepy - less chance of rejection when they are zonked. Or pick the baby up from sleep and put him/her onto the breast and you'll probably find suckling starts immediately.

But all babies like one side best for some reason. DD used to drive me nuts (and actually still does) pointing and saying 'that one'