How old is your little one again? Saggy, empty feeling boobs aren't a good gauge at all, so try not to worry about that one. If that's the case, my boobs would be empty several times over lol, if can actually be really normal as your supply settles and changes over time. The very best indicator of supply dropping are wet and dirty nappies. If that changes drastically, and for a sustained period of time (not just a few one offs) then there might be an issue. Big hugs, this breastfeeding gig is hard work.