Forget the percs. They do nothing. Get some muscle relaxers. If your doc won't prescribe any flexiril or baclofen then just snag some OTC Robaxacet from the pharmacy. Also ice and heat. Hot baths are great. If it is a slipped disc (which it sounds like) try to lay on your stomach for at least ten minutes whenever you can... especially before bed. That will help push the disc back into place (actually pushing yourself upwards creating a curve in your lower back is the full exercise but I don't want you to hurt yourself).
PAY A PHYSIOTHERAPIST to take a look at you and maybe hook you up to TENS unit for a while. Massage therapy is great as well. You do need the MRI and to see a specialist to figure out if and how badly the disc is slipped and which one it is. They may recommend surgery but physiotherapy has the same success rate as the surgery and you are less likely to relapse. It's just a little more time consuming.
I've got a ton more advice I could give you but that's a good start.
To get it down to basics...
Muscle relaxers/Ibuprofen (Advil) NOT percocet or tylonol or aspirin.
Ice then heat then ice then heat.
Get some physiotherapy if at all possible.
Lay perfectly flat and straight for extended periods when possible.
If possible lay flat and straight on your stomach.
Take a loooong hot bath before bed.
Drink lots of water.
...
okay so that might be a little more but that's how you want to deal with this.
Good luck and feel better.