I heard that you should not breastfeed if you have just exercised. Any truth to this?

Nope, no truth to it whatsoever. Exercise, vigourous or light, does not affect the amount or composition of milk that your body produces. The only reason I can think of is maybe because you're tired after exercising and breastfeeding when you're tired could be a wee bit uncomfortable. But that's just my supposition and no scientific reason for that.