Severe pain in the middle of buttock. havnt injured or fallen myself. exercising and stretching make it worse. sneezing makes me almost fall.

Muscular injury: It sounds like you may have injured the muscle. This can happen from overstretching and causing small muscle tears. You should have this checked to make sure that is the problem and discuss appropriate exercise.

Mri?: Sounds like you may have a herniated disc, which can be sensitive to the increased pressure of sneezing or bearing down. This pain generally resolves on its own in a few weeks. If the intensity is too severe, medications or an injection may help. Of course there are also other things that can cause this, and your doctor can help rule or more serious problems.

Disc: Unilateral pain in the buttock that worsens would make me consider a disc problem. Although you are not male you are of the age where bulging disc more likely and when you sneeze this increases intraabdominal pressure and could be pushing the disc out. Get checked.