Don't you have a breast clinic whre you live? This is a subject close to my heart (if you will excuse the pun!) as I have suffered from this for years. I have fibrocystic breasts and every so often the cysts flare up and become almost unbearable! It is hormone-related - it is likely to hurt more just before your period or, as in my case, after the menopause when your hormones are all over the place anyway.
My first serious bout ended up with me at A&E with suspected breast abscesses but it turned out to be cysts, the fluid of which was drawn off by needle - and I can tell you, the relief is immense! To be on the safe side, they gave me a mammogram and a biopsy but everything was fine. The cysts keep coming back, unfortunately.
In your case, I would insist on a referral to a breast clinic. I am fairly sure it will be something that can be sorted out either by diet (no caffeine! little alcohol!) or some medication or treatment and it is unfair of your doctor not to be taking you seriously. As for a muscular problem - I am at a loss to know what muscle he is talking about since most of your muscles are on the chest wall, not in the breast itself!
Don't put up with this any longer - get to a breast clinic and let a specialist see what is going on.