my DD did the same thing sometime between 4-6 months. she did great on my right side and mostly refused my left unless asleep. the only thing that sort of worked was to hold her in a football hold on the left side to try to trick her into thinking it was the right. she eventually outgrew this aversion (within a couple months) and now at one year nurses both sides just fine.
remember that lots of moms only nurse on one side exclusively and produce plenty of milk for their babes (if you think about it, we have two breasts because natural human reproduction rarely produces more than twins!).