following are some suggestions as per your questions...
1. You can not know whether there are sperms in your urine, it could be seminal fluid either. You don't have to get worried about it .as it doesn't have any impact on your sexual or Daily routine life.
2. Stop what...? If you are taking about Masturbation
...then you don't have to worry about the linkage about the Masturbation & your backache
.....you have to evaluate your back issue by Orthopedic specialist.
3. Well in that case you have to again consult your Physician especially Endrologist to get your full evaluation done.
All The best.