The poo is most likely due to the antibiotics and will take several weeks to clear up. As Barb says, it'll sort itself out. If he's feeding and gaining well, then it's probably all fine. DS had similar problems early on when I had antibiotics.
You could also have thrush as well.

Evening irritability, aka arsenic hour(s), is totally normal, though draining, and doesn't reflect your supply or lack thereof at all, as Barb has also said. You'll still be making plenty of milk, your son is just fussy and feeling out of sorts so likes to feed more then to seek comfort. If your substitute bottles for breast then your milk supply may shift, however, to make more when you feed/express and less when you don't. The good news, hopefully, is that you're probably about half way through the worst of the early fussy stage.