Well, I can't venture a guess at why you have ants. But I do have a solution. It's worked well for me and some friends of mine. We mix borax and sugar (about equal amounts) and boil it up with just a little water. Put it in shallow containers (I like to use the lids of jars we're recycling) and set them at strategic spots (like where they're entering the house, if you can tell). It's OK if it dries. It usually works within a couple of hours for me, but I've never had so many as you're describing. My friend tells me that this works better for small ants than for large ones. We always have small one, so I don't know. That friend tells me that what works best for large ants is to mix the borax into meat fat (like bacon grease). I wouldn't really want that in my house, though, so I'd try the sugar mixture first, even for large ants.
HTH
Christie
HTH
Christie