Hi, my 4 month old baby is refusing to feed on ...

Hi, my 4 month old baby is refusing to feed on left side & reduced his feed on the right (I have to pump the remainder) I think it's teething related but struggling to express. I struggle to get much milk per session so yesterday was pumping almost hourly to get enough milk. Not sure what to do? Been going on for almost a week. Shoulders and back aching from all the pumping.