The pharmacist should have put stickers on your bottle with warnings such as possible interaction with alcohol. It sounds like the doc expected him to handle this. I would personally not take this medication often if I didn't need it. I think strengthening your spine by gentle exercise such as walking would be the best thing. Careful use of weight lifting might be helpful if it is supervised by a trainer educated in rehab or an actual physical therapist would be great, I think.
Speculating: It sounds like carrying the packs has aged your spine some by pulling it out of alignment. You could have discs bulging. You don't want to put pressure on the wrong side of your spine and cause them to herniate. Fortunately, most back pain resolves on its own and you won't have to carry the heavy packs now.
Reading your post makes me understand that you need a second opinion possibly from an orthopedic doctor. You may need tests like and MRI, X-rays or CAT scan to diagnose any problems you are having but the test will be worth the time. Taking medication to an existing problem that the doctor has not properly diagnosed without these tests only enforce the 'mistreatment' of a doctor who does not do the proper tests without prescribing medication. That is wrong in my opinion and you should get a second 'more educated' opinion. Doctors who are in a hurry to diagnose or for what ever the reason, fail to see what happen to the body over time with wear and tear.
I had a history of pain issues and it took years to diagnose degenerative joint/disk disease but I spent much time going to different doctors, taking prescribed medication and 12 issues with physical therapy. My disease is not curable but at least I have a diagnosis I can understand. Many of the doctors at the VA explained to me, that if I have surgery, it could result in paralysis. After much research of doctors, I went to an orthopedic surgeon, who performed 5 surgeries and corrected many of the problems I was having. Currently I get nerve block to help with the pain. The nerve doe grow back over time but this procedure helps and if it were not for this orthopedic doctor who has helped with my diagnosis, I could be paralyses if I trusted a doctor who says paralysis could be an issue. That is not a competent doctor. I realize there are 'issues' with surgery but a competent doctor will not scare you with side effects if they know what they are doing.
Do your research and learn about related conditions as described by this doctor but get another opinion from a professional who is not in a hurry to treat with pills. Get the proper tests before a diagnosis is determined.