I think you're right: I think it's muscular. I certainly don't think it's cancer. You didn't say how long you've had this problem, or anything else, but in addition to being muscular, it could well be related to anxiety. If you feel anxious and it seems like your muscles are getting tight, you tend to get even more tense in anticipation of the pain. I'd check with a PCP to rule out anything else, maybe ask for a referral to physical therapy, and try monitor how/when you get particularly tense. Good luck.