I don't know how old your LO is, but if the 5-6 oz was in the first month or two and 3-4 is later, then yes your supply probably went down to match what LO is actually taking. Which is a good thing for both of you! More hindmilk and more comfortable flow for LO, and less risk of the problems associated with oversupply for you. 3-4 oz right after a feed is still a fantastic output.