Hi
Welcome to the MedHelp forum!
The back can hurt because of wrong posture. It can hurt because of a number of other conditions like bone disease, pinched nerve, weak bones, fall on the back, overweight etc. Since you have stretch marks, it could be that you are gaining and losing weight. Since I cannot examine you and since there is not enough information to reach a diagnosis, please consult your PCP.
Hope this helps. Please let me know if there is any thing else and do keep me posted. Take care!
in general, if you have strong stomach muscles, your back is well supported so it won't hurt, however you didnt give much info as to your fitness level, how long your back has been sore or if you did anything to hurt your back.
To help relieve back pain you can lie on the floor with your butt up against a couch/chair and put the calves of your legs on the chair - this will relieve the tension in your back. Sleeping on your stomach isn't good for your back and sometimes if you put a pillow under your knees when you sleep, this could help. Strengthen your stomach muscles - pilates is great for this. However if you've done something to hurt yourself you should go see someone about it.
as for the stretch marks, they are external - I doubt it is anything to worry about.