pulled muscle in grion area where melanoma originated

I have melanoma in the pelvic area.  Have been through treatments 6 weeks ago and going back for a pet scan tomorrow to find out if there was srinkage of tumors.  Two weeks ago a had a bad muscle pull in the back of my leg it has moved around to front in the grion area where the cancer originated.  It doesn't seem to get any better and has been going on about two weeks .  I had an altra sound on it about a week ago and they told me it was and pulled muscle then the pain moved around from back to this area in the grion.  I have had most the the lymph nodes taken from this leg.  I was working out but now of course I cannot.  Do you think this is related to the cancer or the cancer has spread?