There are a lot of forums on here, but I haven't found one dealing with my exact issue. For about two years I noticed my hamstrings felt tight when I ran and my back was a little stiff. I stretched and stretched, and had good flexibility. Never held me back from running. However, over the last 6 months the day after I run, or do any cardio exercise, the back of my legs, mostly right, ache for 2-3 days. It's not a burning or tingling, but a really deep ache. I had an MRI, neurosurgeon says the MRI was weak, but says back looks great, and he's not sure why I am having symptoms. Been doing piriformis stretches and that has helped a little, but not completely. I feel a really good stretch when I internally rotate my leg, but I cannot replicate that aching pain unless I exercise, not just run..... I've thought bulged disc, piriformis syndrome or Si joint issues, but none of the conservative therapies seem to resolve the issue. Any ideas?