Actually, it could be any of the three things you mentioned. You probably strained your low back muscles when you did the exercises and then reinjured it by starting back too soon. Try icing for 24-48 hours only then you can use heat if it helps. Walking is the best exercise to get yourself going without injury. If you want to get an MRI to check for degenerative disk it would tell you what is going on possibly. The MRI should show if you are "out of alignment." Most people don't know what their back pain is caused by though. If it is degenerative disk, you can do specific stretches to help with the disk space. But, I would be careful who I choose as a physical therapist. I was injured twice by PT's and it has taken a couple of years to get back to some level of normal. They are not regulated and all are different. I wouldn't see a chiropractor nor have anyone adjust you other than a doctor. A pinched nerve usually radiates down the leg and feels like burning, tingling or numbness. Sometimes weakness in one leg.
80% to 90% of individuals in the US, and maybe the world, have some degree of low back pain during their lifetime. I doubt if humans evolved to have such a problem and the thousands of people, yearly, who have spinal surgery have a very bad rate of having a meaningful improvement a few days or a few weeks after surgery. If people get better in months or years, ort on their own, as you often do, that in itself is enough to bring questions into the paradigm of: low back pain=herniated discs. If it was a herniated spinal disc that was causing the problem spinal surgeons could fix it 97% of theg time, but their rate of helping people within a few days to a few weeks of surgery is: dismal!
That is what I decided after over 25 years of being a physician and "doing what I was taught", so to speak, but after so many surgical failures and repeated surgery, one guy had 8 back surgeries, for instance I decided there was something "rotten about the who scenario."
After a two and one half year clinical investigation during which I attracted 700 miserable patients who had low-back pain and/or shoulder/shouler pain, with tingling and numbness to the appropriate limb, I found out the cause of "most peoples' low back and/or shoulder/cervical pain: autoimmune neuropathy caused by streptococcal infections. Most people had had tonsillitis, strep throat, scarlet fever, rheumatic fever, and flu-like diseases wherein they had lethargy, body pain, fever, gastrointestinal problems, slept alot....like for two or three days, etc. Some people had "skin pain" during the disease so they could not even stand sheets on their body.
They all had red palms, red plantar surfaces of the feet, red around the finger nails and the knuckles, some sign of arthritis, some people had been diagnosed with fibromyalgia, some people had had alot of heartburn, some people had had hypertension, some people had had carpal tunnel syndrome diagnosed, some women had sexual malfunction, that is difficult orgasms, some people had lupus erythematosis diagnosed in the past. Some people had had ulcerative colitis or cholecystitis and their gall bladder removed.
As it turns out the autoimmune disease is systemic and it attacks all tissues and, therefore, all organs in the body and over time it can cause "lots of problems; the back pain is just a warning.
O.K. if the above sounds like you, well drop a line.