If there is nothing broke or injured, such as a disk, you most likely have vertebrae out of place.
Since I have a specific problem with this, I have learned how to tell if I have just strained the muscles or if I have a vertebrae out of place. In my case it is always the L-2 that slips out of place. I have also learned how to put it back into place myself. About 75% of the time I can do this. If I cannot get it into place, I have to go to a Chiropractor. Any of them can do this, as it is a common problem.
I was in the moving business, so this was not an uncommon occurrence. It is not so much related to how much you are lifting, as it is the position you are in when you are lifting. That is another way you can tell if it is strain related to vertebrae out of place or just a simple strain. If you were not lifting anything far heavier than you normally can handle, then it is most likely a slipped vertebrae. The position made it happen, not the weight.
If you have eliminated a bulging disk or fracture, a normal back strain should heal in about a week. In another words, you should notice improvement in that time.
If I go over that week time frame, I have a vertebrae out of place. As long as it is out of place, you have muscles that are stretched out of place, and the pain will not get better. After the Chiropractic adjustment, it will feel more sore the next day, and after that it will get progressively better. One week after the adjustment, you should be noticeably better, close to normal.
In 15 minutes they will be able to tell you if this is the case or not. Tell them you have osteoporosis, as there are different methods to line up the vertebrae, and some are gentle, and others are more forceful. I tell them to use the gentle method, as I do not like someone wrenching on my back.
Pay attention to how they do this, as you can duplicate it at home if this becomes a reoccurring problem, as it is for me. The gentle method can be duplicated, I would not try the forceful method, as you could really harm someone.
If the Chiropractor cannot help you, then you need to have a some more specific tests to find out what the problem is. Most back problems are simple strains or misalignment's, so that is a good place to start.
Dan