Probably it's your baby cutting down on nursing. I haven't hit 18 months without being pregnant yet, so I can't swear to it.
Milk, as I'm sure you know, is a supply-and-demand issue, & if you're down to only a couple of feedings a day, unless they're incredibly long marathon feedings, I wouldn't expect to have a "full" supply still. How it affects your calories, I honestly don't know.