Hi!
It sounds to me as though you may have developed a cyst in your breast. It may be too deep within the tissue to distinguish on self exam. If you were my patient, I would do a breast ultrasound. A mammogram is not going to be very useful in someone as young as you are who has never had children--the breast tissue will be too dense.
If the breast ultrasound does show a cyst, it can be aspirated under ultrasound guidance, and cured. If the ultasound was negative, I would give you the standard speech about: reduce caffeine, use anti-inflammatories (motrin) as needed, make sure you have a good support bra, and contine to do self exams.
I hope this is helpful!
Good luck!
Dr B
also my cycle usually runs 26 days apart (once in a while a few more days)...