If you're taking cold medicine, that is your problem. Antihistamines and decongestants will decrease your milk supply. Also, just as you always hear that you should drink plenty of fluids when you have a cold, it is that much more important to stay hydrated when you are both sick and nursing. Be sure to drink lots and lots of ice water.
Anything that dries up sinuses can dry up your milk so if you are taking something then stop. If you are not then make sure you are getting plenty of fluids to keep supply up. When my son was 3 months old I got a TERRIBLE flu and ear infection and my FIL asked if I should still be nursing and my wonderful husband said "of course dad that is the best thing for the kid because all the antibodies she is making are passing to him in the milk". So make sure to take care of yourself and eat and drink as much as possible and you and LO will be just fine.