owch!! not good, i love attack also and at times when i have overtrained (naughty really) and my body is screaming at me ( my piriformis like to yell at me).. i find that warming up on a bike and stretching well before really helps!!! i also now go to see a sports masser, she is ace and that has helped so much!!!!!
heaps of things can cause it, but you should always be mindful that technique is right.. please dont think im suggesting you do have bad technique as i have no idea at all.. just check your grounding your heels as if you are doing it a lot on your toes your poor calves will be screaming at you!!!
if it is real bad get yourself to the docs just to be safe!!!
hope it gets better soon :-)