Piriformis is a short, deep glute muscle that lies underneath the large Gluteus Maximus muscle. Piriformis muscle pain can be excruciating and make everyday activities difficult. Don't take my word for it - the comedian Sinbad has a bit on the pains that Piriformis can cause.
Piriformis Muscle (red) from a Posterior View
What does Piriformis muscle pain feel like?
Piriformis muscle pain refers to the tailbone and outer buttock regions. In the left image below from Travell & Simons Myofascial Pain and Disfunction, the red areas represent where pain from Piriformis is typically felt .
That being said, there is another pain pattern the Piriformis muscle can cause - sciatic nerve pain down the leg. This phenomena, known as Piriformis Syndrome, is triggered when the Piriformis muscle is so tight that it compresses on the large sciatic nerve that runs from the base of the spine down the leg . This can be extremely painful... trigger point therapy and stretching (in that order) is a very important for patients with Piriformis Syndrome.
Piriformis Muscle Pain Pattern (left) and Piriformis Muscle (right) 
How can I relieve Piriformis muscle pain?
Piriformis muscle pain relief requires ridding it of trigger points and lengthening the muscle - in that order. Stretching is very difficult when a muscle contains knots/trigger points, so I'd highly recommend starting with trigger point massage.
Piriformis trigger point relief can be done by your physical or massage therapist, using a lacrosse ball, or with a QL Claw device. I prefer the QL Claw for this function because a) I don't love my PT digging around in that area, b) The QL Claw stays on the ground and gets in there deep, and c) QL Claw was designed with Piriformis specifically in mind. QL Claw was designed to release the 5 muscles that contribute to the majority of low back, hip, and glute pain - and Piriformis was certainly on that list. Check out QL Claw's Piriformis release tutorials and own your Piriformis muscle pain today!
 Donnelly, Joseph M. Travell, Simons & Simons Myofascial Pain and Dysfunction: the Trigger Point Manual. 3rd ed., Wolters Kluwer Health, 2019.
 Davies, Clair, and Amber Davies. The Trigger Point Therapy Workbook: Your Self-Treatment Guide for Pain Relief. 3rd ed., New Harbinger Publications, Inc., 2013.