Lumps between/under breast. Correct referral?

Hi there, I am new to this forum. A few years ago I had a lump under/between my breasts. It didn't hurt or bother me particularly but I mentioned it at my routine scan. The image didn't cover the area where the lump was so I was referred to a breast clinic. The doctor there was clear that it was simply cartilage on the end of a rib and nothing to worry about. In more recent times the lump has spread and is quite painful. I went to the GP who has not seen anything like it before. She thought it may be a sebaceous cyst but referred me to radiology for an ultrasound scan. They refused the referral and said the breast clinic should see me. The breast clinic refused and said a chest wall lump is not their area. Now the GP is referring me to a dermatologist. A month has passed since I saw the GP and I am no further forward. Has anyone else had this experience? I just want to know what referral is best. The lump is on the bra line and uncomfortable much of the time. I never wear underwired bras but even light sports bras cause tenderness on the lump. Any thoughts are much appreciated. Many thanks.