i'd probably see a doctor about it. as cindy said, if blood clot, then dangerous.
do you use muscle relaxants, does that make it better? does stretching make it better? does hot or cold make it better? if so, then it sounds more like a tight muscle. mine get tight for the reasons cemc described, but these things do helps some. and it gets especially tight if 1) i'm walking a lot and start getting bad PF or 2) i get an AM calf cramp.
Spondyloarthropathy, HLAB27 negative
Humira (still methylprednisone for flares, just not as often. Aleve if needed, rarely.)
LDN/zanaflex/flector patches over SI/ice
vits C, D. probiotics. fish oil. CoQ, Mg, Ca, K.
no dairy (casein sensitivity), limited eggs