Brian, this is a long shot but I'm going to tell you about it on the chance that it might help in some way. First: I'm going to assume that you are a cashier or something similar where you can't get to the men's room for several hours at a time. Second: I'm going to assume that you don't drink a lot of water for that reason. In some people, like myself, if they don't keep drinking plenty of water, they can be more susceptible to lower back strain and pain.
I'll use myself as an example: Just a few days ago I decided to drink less water so that I might not have to get up during the night to go to the bathroom. It did help in that regard but in the morning I woke up with a strain in my lower back. I found it difficult to straighten up without hurting. I was wondering why I should have that problem and then, suddenly, I remerabered about the water. So I began drinking a lot of water to rehydrate myself and my back strain went away like magic.
It wasn't the first time this happened to me but it hadn't happened for a nuraber of years so I needed this to happen as a reminder.
If this works for you, tell your boss about it and let them know that you need a bathroom break every so often because you need to drink plenty of water.
Good luck.