Hi S., my 5 yr. old son does the same thing. Sometimes I think it's because he had a bad dream or that he's cold and not waking up enough to cover up again. Whatever the reason, half of the time I don't even wake up enough to put up much of a protest and I don't mind except when he sleeps with his feet all over me, then I can't sleep. The only thing that has worked for me in the past is to get back up with the child and put them back in bed and stay with them till they fall asleep. This works pretty good when I'm consistent, it takes like 3-4 nights of this at least. I think my son does this when things wake him up(bad dream, cold, have to go potty, etc.) and then his sleep pattern gets mixed up and the only thing that makes him feel better is mommy. They eventually grow out of it, my oldest is 19 and he's a "flopper" and I survived him, sleepily I might add. :)
I am more concerned about you and your back problem, you might need some physical therapy and such to get back on track. Please take care of yourself, I was a single mom for 13 years of two kids and now am a married mom of 6. Taking care of yourself is the greatest and HARDEST and MOST necessary thing to do. If your son isn't causing you to loose sleep, I'd just let him sleep with you. Hope this makes you feel better, Good Luck with the schooling, Good Job!!!!
M.