Yoga can literally be a "pain in the butt." Particularly with poses that require your gluteus maximus muscle to work hard or poses that overstretch this muscle. Yoga poses might be uncomfortable, but they shouldn't cause pain.
Read more: Sore Muscles After Yoga
Pain can be caused by poses that require you to lift your leg backward -- a motion that uses your gluteus maximus muscle. This muscle also works hard when you are balancing on one leg. If this muscle is weak, you could have pain as the muscle becomes fatigued.
For example, Warrior III is performed by lifting one leg straight out behind you as your torso bends forward until your body is parallel to the ground. Your body weight is supported on one leg, requiring your gluteus maximus to work on both sides.
Pain from muscle weakness shouldn't keep you from practicing yoga. All poses can be held for shorter amounts of time until your strength increases. Modifications can also be made until you are able to do these poses without pain.
For example, perform Warrior III while standing with your back to a wall, approximately one leg's length away. As your torso moves forward, lift your leg behind you and rest your foot on the wall with your toes pointed down.
Read more: What Are the Benefits of Warriors in Yoga?
Yoga poses improve flexibility. However, pain can occur if you overstretch a muscle. The gluteus maximus muscle is stretched with any poses that require you to fold over at the hips -- such as the Seated Forward Bend. This pose is performed by sitting with your legs straight out in front of you, bending at the hips and resting your chest on your thighs. Stretching might feel like a strong pulling sensation or mild burning sensation, but you shouldn't have sharp pain.
Modify poses to decrease tension on your gluteus maximus until your flexibility improves. For example, perform the Seated Forward Bend while sitting on a folded blanket to decrease tension on the back of your hips. Or loop a small towel around the balls of your feet and hold one end in each hand for leverage as you fold forward until you feel a stretch in your gluteus maximus.
Read more: How to Stretch the Gluteus Maximus
Although you might have pain in the area of your gluteus maximus, the problem could be a compressed nerve in a muscle just below it called the piriformis. This condition -- called sciatica -- typically causes pain that also runs down the back of the thigh. It also causes numbness or tingling in the same areas. Buttock pain can also be caused by pinched nerves in the lower back.
Consider seeing a medical professional for an accurate diagnosis to help you decide the best way to modify your yoga practice.