You had your hands behind your head while sprinting?
On the cleat thing, gyms never seem to service their bikes properly. I usually take my Topeak multi tool and tighten the spring on the bike cleat before using them.
Its easy enough to work out I this is muscle of nerve.
Side bend - If you stand up and run your fingers down the side of your leg on each side can you get as far on both sides?
Does it hurt if you do a standing forward fold?
Can you do an up dog pose (yoga)?
laying on your back with your legs out straight can you raise them up? does it cause pain?
If these don't cause any pain then it's likely to be a pulled muscle. If they hurt like f** then you might have caused a herniation in your spine, particularly if you've had sciatica before.
Either way its going to be worse tomorrow.