Though the definitive root of your back pain may be murky, your immediate course of action is clear.
Back off strenuous activity, but stay mobile. Trying to override the pain by ignoring it won't do any good, but neither will spending a week in bed. If you let your pain be your guide — do what you're able to do without making it hurt — you won't feel so stiff. Get up and move around, even if it's difficult. Perform a few exercises. If you are incapacitated, stay in the sack, but for no more than 24 to 48 hours.
Apply ice and heat. Ice is particularly therapeutic for acute spasms and inflammation. When your back goes, ice it immediately and follow up two to three times a day for the first several days — more often if you need it. (A bag of frozen peas works great, because it conforms to your body.) After 72 hours, if the pain persists, you might try contrast therapy, in which you apply ice for ten minutes and then take a hot bath with Epsom salts or apply a hot pack.
Take mild pain relievers. Aspirin and nonsteroidal anti-inflammatory drugs, such as ibuprofen, will help reduce inflammation and thus pain. If it persists without improvement for a week, seek help.