It could be a slipped disc, but its not just a slipped disc but many other problems with the spine, like osteophytes or spondylosis (decreaed space between two vertebrae) which could cause this pain. An Orthopaedician will be able to diagnose this, that is why you need to consult one.
Besides exercise, there are many other methods. Some of them are Lumbar belts, Traction, etc. These would be advised by the orthopaedician and are likely to cure you.
It is because of the back pain and the muscle spasms of the back muscles that you are not able to stand up straight. Once the pain is gone, you will be able to stand up again.
Although giving up the job may not be an option, you can take some steps like sitting down whenever you can, change to a different counter like the cash counter where you may not have to stand so much, or at least not bend so much,etc.
I hope this answer was helpful. If you need more information, feel free to ask. For accepting this answer, please click on the green 'accept' button below. Feedback/Bonus will be warmly appreciated.
