Agree with Marcus, could be that you have a weak core. Since running up hill you adopt a different posture which could put strain on your back.
As stokey said no harm in seeking help. Speak to your GP. My bro has suffered from a bad back for a long time and after months of physio they finally told him that its probably just a weak core. Unfortunately the lemon doesn't do any core exercises so i can only assume he still suffers...
If not the core then it could also be your hamstrings. My Handball coach works in sports science and says tension in the hamstrings pulls down on the back, leading to back pain and, in later life, proper back issues.
Get on the floor, straight legs, and try and touch your toes. Apparently you should be able to reach across with one arm to the opposite leg and pick your foot up, holding it in the air for 10 secs. If not, work on it. I had major flexibility issues and couldn't reach my toes, had a few back problems but now I can reach my feet and back is sorted.