Hey mate, I see patients with back spasms on an almost weekly basis. It will normally go away by itself. If you want to help it on its way then simple combinations of paracetamol and Ibuprofen will do the trick. 2 tablets of paracetamol every 4-6 hours, 2 tablets of ibuprofen 200mg 3 times daily with food.
Start concervative and work your way up, if those meds don't help then get some analgesic calmative, it has paracetamol 500mg, codeine 8mg and doxylamine succinate around 4mg. The doxylamine is an antihistamine muscle relaxant and will do the trick. This is available over the counter, the more expensive version is mersyndol night.
Voltaren gel may also help if you can get someone to massage it in.
Oh and don't just stay idle all day, get up and move around and stretch it out. This is one injury that is beneficial to keep moving.
Cheers.