Soft tissue injuries are tough - they can take a while to heal. They can occur with very simple motions - bending over to spit out the toothpaste, that sort of thing.
If it's in your butt and maybe going down your leg, it may well be sciatic nerve pain - but that can be affected by a number of back muscles and SI injuries.
Stop stretching for right now - do ICE! 20 minutes on, 20 minutes off. Try an anti-inflammatory, either over the counter or a SAFE natural alternative (not just anything that says "natural" is safe or wise). Rest. Lie down with the small of your back into the floor, knees bent.
My husband just went through this - and he is extremely athletic, in shape, etc. But he did some exercises the wrong way, without proper support, and that's all it took. He did medicinal doses of anti-inflammatories plus a natural product that does not conflict or have side effects. It took about 10 days. He did use a prescription muscle relaxant for a few days, per the doctor.
If you don't rest, this could take a lot longer. Let the cardio go for right now, even walking - if you keep reinsuring yourself, you'll take forever to get well. The more you increase the inflammation, the worse you're going to make this. And the pain often moves because when you are hurt, you either consciously or unconsciously adjust your posture and carriage and activities to favor the injured area - and that puts strain on new muscles. You're better off just really healing up, and trusting that you will regain your cardio benefit and other muscle development/workouts (as well as letting your floors go a week or two) if you just wait it out.