Hello,
The following is a quote from my "Healthy Moms Perinatal Fitness Instructor Training Manual:"
Lactic Acid, Exercise and Breastmilk
Studies now show that regular, sustained and vigorous aerobic exercise at moderate to high intensity does not alter the quality or quantity of breastmilk. Unless exercise intensity is very high (above the anaerobic threshold), there is little change in lactate levels in either maternal blood or breastmilk.
In other words you should be fine to nurse after your vigorous workout. For example, Healthy Moms runs four high intensity "Powerstroll" classes each week. After each class, almost all of our nursing moms feed their babies. I have never seen a baby get "tuned off" by the breastmilk of a mom who has just finished one of our stroller fitness classes.