Exercise has not been found to affect a mother's milk supply and studies find that there is not an increase in lactic acid in breastmilk during moderate exercise. However, when exercising at maximum intensity there will be an increase in lactic acid for up to 90 minutes after terminating exercise. There have been no negative effects found for babies that take in increased lactic acid in breastmilk. Therefore, there is no reason to wait after exercising, even at maximum intensity, to breastfeed your baby.
You may wish to breastfeed just prior to exercising for comfort purposes and to wear a supportive bra. If you find that you have an increased amount of blocked ducts, you may want to cut down on upper bodywork where arms move frequently (i.e weightlifting). You should also check to make sure your bra fits properly and is not compressing the breasts.
Some babies may not like the taste of mother's sweat, so you may wish to take a shower after exercising and before feeding the baby.