Is this back issue serious?

See a doctor if you think it is important and severe.

Keep in mind that your kidneys are in your lower back. If you have any urinary changes,or weight gain, you should get some blood work. Especially if you recently started new meds.

Probably just a sprain….

Prolly sciatica, but it could be pleurisy. Check with Dr. to be sure. A 50-dollar X-ray will rule out the latter.

First step for me would be to visit a chiropractor. Simple adjustments are very helpful, and it’s completely possible that you could just have something out of place.

When I went in with something similar, I was told to roll up and duct tape two towels. Put one vertically along your spine, and put the other horizontally beneath your neck. Lay your arms in a T position and breathe or just chill for 10 minutes a day,
Try both for about a week. Chiropractics first- they’ll tell you if it’s something more than posture or placement.

Chiropractors are not doctors.

I would consult a physician first if it comes to it.