You could cheaply purchase epsom salts and find peppermint oil in your cooking isle (do not get synthetic). Combine the two in a bowl and go easy on the peppermint...it is a stimulant. Draw a very warm bath and dissolve the oil and salt mixture completely then soak. This will draw blood to your back area and loosen the muscles. Also, stretching daily (bending over and reaching as far as you can and doing long slow twists at the waist) will alleviate muscle stress of the back. *I am not a doctor or herbalist. I have used the remedy however if you are allergic to peppermint oil or use too much it could go badly for you. Take this advice at your own risk*
If you have access to a Willow tree then the bark can help. There is a compound in the tree which commercial aspirin is modeled from. A tea brewed along with the bark should help ease the pain, although if you can't use aspirin for whatever reason then this restriction would likely stand with the natural compound in the willow also.
There are some teas like chamomile which would help relax muscles, so that if the cause of the pain is muscle tension and stressed/achy muscle then these kinds of herbal tea may help.
Cinnamon was used as a cure-all, but I haven't personally seen anything indicating it is good for pain.
I can't think of anything else off of the top of my head.
Be careful with what you use because you could seriously do some bad if you're not paying attention. Research all of the medical and scientific aspects of the plant before you use it, so you don't accidentally use something psychoactive (trippy, in terms people use today).
Also, some plants can be poisonous so you have to check. With some plants, only a certain part is poisonous, with others, it is the entire plant.
You could utilize mint. Buying peppermint or plan mint tea (again the real thing). Take a couple of the bags and seep them in boiling water until the tea is very strong (too strong to drink). Add the Epsom salts to this and then utilize the concoction for the bath. Peppermint is really the best for this type of issue.
Non Kitchen Witchy stuff to utilize would be Vervirite, boneset and wild dandelion roots.
Get yourself unprocessed grain. Put the grain in some metal bowel or some sort of fire proof plate and place it in the oven. dont bake it or burn it just make it warm and pleasantly hotish, than take it out and make sure its not hot and while still warm place it in some cloth and put it under ur back and legs. Do this at night while going to bed and sleep on it for about a week and if it doesn't work feel free to curse me ;)