I have a blood blister that appears on my shoulder the week before my mental cycle, and then disappears after each month. what is it?

Poss. endometriosis: Based on your history, is is possible that you have an extra-pelvic (outside of the pelvis) endometriosis. This would be rare but I have seen one case (ironically) in the shoulder joint approximately 18 years ago. See your doctor for confirmation and a possible biopsy.

Menstrual cycle rash: Endometriosis of the skin is rare but does occur and the lesions can bleed and cycle. They are under the hormonal influences as is the endometrium of the uterus. There might be other things such as vascular growths that can also be affected by hormonal fluctuations, but such an unusual lesion should be biopsied during its peak activity to have a dermatopathologist confrim it is of no concern.