A minor drop in milk supply is normal following an illness, especially if you were dehydrated. Nursing (and/or pumping) more frequently for a few days can help boost your supply. If you have a cold, steer clear of antihistamines — they are known to reduce milk supply. Menthol cough drops can hurt your supply as well.
If your supply has been lowered significantly and frequent breastfeeding doesn't bring it back up, talk with a lactation consultant about other ways to raise your milk supply.