Sounds like the pump to me, could be the filter fell off or is clogged as well. Probably the pump though. They aren't herd to replace, if it still shifts okay or did before, you could try replacing it. If it was slipping before or is now and you have driven it a lot while slipping, then there's no point. It's done.
The transmission pan itself would be a good indicator, too. If there's metal in it (more than usual), then there's no point. It's done.