Straining (Muscle pain) in Sacrum region

Not sure if this is the right forum but examing the forum list suggested that this was the closest forum.  

I'm Male, Late 20's. 102kg, 5'11, 36 inch waist. Although I work at a desk job 8-9 hours a day,  I workout everyday for around 20-30 minutes (generally both anaerobic and aerobic) and have been going on Hikes roughly every month for the last 2 years.  A problem comes every time I attempt to hike up steep hills where I start developing excruciating pain around the sacrum region, beginning around my upper buttocks (possibly including the lower lumber region), then radiating to my hips.  The strain eventually gets too much to bare even when I'm hiking up at a deliberately slow pace and I find I still eventually have to stop.  Although I'm able to hike very fast on the flat, I often fall Way behind the group on steeper up hill sections because of the pain (To put it perspective, Many people who are much fatter than I am (as in, they possess a large pot belly), have only just started hiking and are older than me, are able to beat me up a hill).

Has anyone experiencing something like this before?  
I am flat footed, but never had any pain associated with my feet, however could this contribute to pain in the lumber / upper buttocks / Hip region when negotiating hills?
I've trialled hiking poles for several hikes before, but found they didn't really assist that much (it at all).
Would performing exercises that target the sacrum region improve this?  

Many Thanks.