I am 25 and since I started on a new brand of Pill, I have had awful pain at the top of both breasts.

My doctor says it is "mastitis". However, I'm really terrified I may have cancer.

Relax. Cancer is very rare at your age and it does not produce these symptoms. Mastitis means "inflammation of the breast tissue", and it's very common. It is often influenced by hormones, so my guess is you probably need to change to another brand of Pill, if your doc agrees.