4 mo has gone off one boob

[Sugarcoma]

I tend to feed DS more on my right boob, I think because it's easier to position him with his head on my right arm, especially when in public. For that reason my right boob has always been a little bit fuller although I do try to alternate him as much as poss.

In the last couple of weeks he's really gone off my right boob - he won't stay on as long when I do get him on and cries and gets distressed. He then calms down and drinks happily from the other one.

I've now found some lumps in my right breast which I assume are blocked milk ducts.

Any advice on why he might have gone off one boob? Anything I can do to get him back on? And any advice on sorting out the lumps? ATM I am using a hot water bottle and trying to hand express a bit into a muslin.