If your profile is correct on your location (NY) then that was illegal. I would call the store back and demand to speak to the manager. Also call local news stations, and LLL reps. VERY illegal, and if you don't stand up to them they will do it again to another momma :(
Just saw on your profile you're in CNY - me too! - but legally, they have NO right whatsoever. NY has some of the most protective laws when it comes to breastfeeding mothers. You could have sat there, taken your shirt off and breastfed, and they legally couldn't do anything.