It's absolutely possible to feed off one side only. Think about a mother of twins, she will feed the equivalent of one baby from each side, often without issue. However it is definitely worth seeing a lactation consultant as if your breast is not changing in pregnancy then this might indicate an issue with the breast tissue on that side (only might, it could be a heap of other things too). Have a google of "insufficient glandular tissue" and see if you feel this fits your case.