I am not sure about this - Lower Left Side Muscle Pain

I have relapsing remitting MS and suffer from a lower back left muscle pain, only when i am sat on the floor in the early morning sorting out the daily deliveries for various customers or the shop. Just lately, due to the cold weather and finding out that side has caused a lot of pain i have begun to wear a hot water bottle on that site.

Is this related to MS or is it my stupid back being a pain as i have pulled muscle in my lower back before, not related to MS.