While we wait for your responses to Say's questions to get more information about your pain, you may want to read the following about "pyriformis syndrome" to see if this is what you may have:
"Piriformis Syndrome Signs and Symptoms
Piriformis syndrome usually starts with pain, tingling, or numbness in the buttocks.
Pain can be severe and extend down the length of the sciatic nerve (called sciatica). The pain is due to the piriformis muscle compressing the sciatic nerve, such as while sitting on a car seat or running. Pain may also be triggered while climbing stairs, applying firm pressure directly over the piriformis muscle, or sitting for long periods of time. Most cases of sciatica, however, are not due to piriformis syndrome."
The following is what Suzanna McGee, who posts here as sixftlion, says on her tennis fitness website:
"Sciatica pain can be close to paralyzing—a numb, tingling or burning sensation going down in the back of your leg, calf and sole of your foot. The pain comes and goes unpredictably and playing tennis becomes impossible. However, you actually could have a tight and overused piriformis muscle, which is very common among tennis players.
Luckily, it is also easy to correct.
Piriformis is the largest of the six muscles in the hip that are responsible for external rotation of the leg
. If you have your leg planted, the piriformis turns the body in the opposite direction, a movement that tennis players do repeatedly thousands of times. An overused and tight piriformis muscle causes a lot of misery and pain in your sacrum, glutes and hips. It will twist your sacrum a little bit, causing a short-leg syndrome that adds to the problem. It can also compress the sciatic nerve and as a result, you feel the “sciatica” pain.
Quick directional changes in tennis impose a high risk on your piriformis’ well-being, especially if you are not well conditioned. Therefore, it is important to work on strengthening your glutes and hips, accompanied by regular stretching.
Prolonged inactivity or sitting puts the piriformis muscles in trouble. If you sit at work or school most of your day and then start sprinting around the tennis courts, you may be creating future problems. An overused, shortened and sometimes even inflamed piriformis muscle contains painful trigger points. To remain pain-free, you need to stretch the piriformis to its original length and eliminate the trigger points with myofascial release."
You can read more about it, especially about how to treat it, by clicking on the above URL.
Here is a little more information why tennis players are more likely to develop the pyriformis syndrome if they do not do off court conditioning.
"Runners, bicyclists and other athletes engaging in forward-moving activities are particularly susceptible to developing piriformis syndrome if they do not engage in lateral stretching and strengthening exercises. When not balanced by lateral movement of the legs, repeated forward movements can lead to disproportionately weak hip abductors and tight adductors. Thus, disproportionately weak hip abductors/gluteus medius muscles, combined with very tight adductor muscles, can cause the piriformis muscle to shorten and severely contract. Upon a 40% increase in piriformis size, sciatic nerve impingement is inevitable. This means the abductors on the outside cannot work properly and strain is put on the piriformis."