KatiKat - the pain in your muscle is probably the muscle. I get horrible muscle spasms, cramps and contractions - and pain.
A Mom - Sorry about your son's diagnosis. As someone else mentioned, inflammation affects everything - including smooth muscle. I tried increasing potassium in my diet and taking potassium supplements and it helped a little bit. Right now I'm adding vitamin D to my supplements because I'm low (a blood test is needed because too much Vit D is a bad thing, too.)
Ask the doctors if they've done a lupus panel. It's a HUGE blood test - many vials of blood. The reason I ask is because my mom had blood clots. One of her earliest symptoms was what felt like a rapid, irregular or skipped heartbeat. Doctors hooked her up to a monitor and could find nothing wrong with her heart. Turns out a blood clot in the lungs can feel like an arrythmia. A lupus panel can reveal (1) more information on the AS since lupus, Crohns disease and spondylitis are related diseases and (2) a genetic factor that increases the risk of developing blood clots (I'm positive for that genetic factor).
Diagnosed May, 2004 with undifferentiate spondy. Now have AS diagnosis. HLA-B27 negative female with no family history of spondy. Taking Mobic and Cymbalta. Discontinued Enbrel, Oct 2005.