I agree with dinoparty. I have piriformis syndrome, complicated by the fact that my sciatic nerve goes right through that muscle. When the sciatic nerve gets irritated, the pain shoots down my leg to the ankle. The orthopedic doctor gave me these stretching exercises, which really helps. For pain, I take generic ibuprofin.
Everything else witten here will help. It is vital to get an image and make sure it isn't something more serious than just muscle spasms. Find a sports medicine practice with a PT or MT with real Neuromuscular Massage Therapy certification. In combo with everything else, this will help you through the bouts of muscle spasm.
For daily low level moist heat, try ThermaCare, the oxidizing heat wraps. 2 for $7 at Walmart. I swear by them. Place the heat-producing portion over the area of the upper buttock that hurts the most. You can shift the wrap during the day as the tightness shifts around the upper glutes/low back/hip. They last 8-10 hours.
Until you can get the massage, which will relax and release the psoas (anterior hip flexor), keep your back straight, like riding a horse and bend at the knees, not the waist. Move your weight over your hips. In Tai Chi, this is called Horse Stance http://www.wikihow.com/Do-Tai-Chi%27s-Horse-Stance and it is best to use this without necessarily having your arms outstretched whenever you need to bend forward, such as making a bed, brushing your teeth, wiping a counter, speaking to a child or someone shorter than you. To do the activities without injuring the involved muscles, use a variant of the Tai Chi Grinding Corn exercise http://www.ehow.com/how_11968_perform-grinding-corn.html The pics show a forward bend. Don't do that. Keep the back straight and the legs bent and go about the activity. Your thighs will become strong, as well. That slight forward bend at the waist can aggravate the pain and if your core is weak, just a sneeze while bending forward at the waist can spasm the psoas. Common triggers are picking up a vacuum cleaner or a small child or animal while bending forward at the waist. The pain will refer to the low back, but the problem originates in the psoas. We flex this muscle hundreds of times a day, but it is one muscle that never bulks. It begins to weaken in your mid-thirties.
The condition is chronic-remitting. It will come and go. Everything mentioned, especially too much sitting and cold damp weather, will set it off. Once you have a set of exercises/stretches and a PT/MT/chiro who help you, see them at the first twinge to stave off increasing pain, misery and disability.
It is very common, as you can see from this thread.