I sprained the muscles in my lower back and can't
Prescription medications are more effective than over the counter variants.
Severe back pain are better addressed with prescription NSAID painkillers along with muscle relaxants. Without clinical examination, I am not allowed to send a prescription here. If you are unable to see a doctor early, I would suggest to try urgent care clinic or ER for a prescription.
In the meanwhile, you might apply topical pain relief agents (spray) followed by moist heat / warm compresses in addition to tylenol extra strength + ibuprofen.
Hope this helps. Let me know if you need clarifications.