What to do with my pain? (near the joint between my backside and leg. towards the left)?

I had similar pain caused by sitting in the back seat of a car with an erratic driver. My back gave out a couple of days later because the pain was right above my hip, near my spine, on the right. I still feel tender if I sleep wrong or don't stretch. I had to go for physical therapy. When one muscle in your back is hurt, no matter how small, the muscles around it "guard" and cause more pain. Stretching is all that works. Biking will only aggravate it, so definitely stretch significantly before a trip. The only stretch that works for me is sitting on a chair, reaching towards the opposite side of the pain (my right side hurts, so I reach to the left), grab the leg of the chair, and pull by raising your top shoulder. I'm flexible, so moving my legs around to try to stretch this spot doesn't even help. Good luck.