Have you even looked at the fluid level yet? If it's consistently slipping the friction discs in the clutch packs have probably already burnt off the friction coatings on some of the discs and you really might be looking at a trans rebuild to actually fix the problem. I'd tell you to stop driving it but it sounds like the damage is done. This is probably why Iminhell said what he did above.
Check the fluid, if it smells really burnt and nasty you've got problems that fresh fluid won't fix.