Get in to see your primary care physician ASAP. They can do an examination and any necessary tests, and determine what is likely causing your problem and refer you if needed to a specialist, either now or later. Hopefully your problem is just a nerve being pinched due to a knotted muscle and not a problem in your spine such as a herniated disc, but the doctor will need to do testing to find out for sure.
Meanwhile, ice on your lower back 20 minutes at a time a few times a day, ibuprofin if you can take it, and avoiding lifting and excessive bending should help relieve some of your pain. Try to get up and walk around or change positions frequently as much as you can tolerate and don't stay in bed.
Warm wishes!