Guest, (I'm not a doctor, just a guitar player and husband) I believe the are you describe is a common area for women to gather 'stress'. That is, the muscles in this area are tensed in order to control some stress factor. We have had some success with daily massage, but one should look at factors in the environment to eliminate. For example, is there someone new in your band who requires extra effort to keep on beat? If you and your Bass player are not easily locking in are you having to force the beat? Are you singing more now and therefore under more pressure while playing?

I agree with seeing your doctor, anti-inflammatories would probably help and some rest to let inflammation in the muscles heal. Now I hate to say this, but I am 55 and I can't play or 5 or 6 hours straight without breaks any more. So young as you are maybe some breaks and stretches for your shoulder are in order.