To put your mind at rest, the chances this is breast cancer is pretty low given that you are healthy and young. Pain is not a common early symptom of breast cancer. That being said, any persistent symptoms should be evaluated by your physician.
The most likely cause of this aching pain is a strained muscles. This could occur for a variety of reasons, including lifting heavy objects, sleeping in a funny position, or exercising vigorously. Resting the area and taking an anti inflammatory medication may help.
Another possibility is that the ache is from stretching of the breast tissue itself. This is common especially in women with larger breasts and may require making sure that your bra is properly supporting the breasts.
Again, cyclical breast tenderness is common, especially that comes and goes about a week or so before the menstruation. This can be alleviated with rest, avoiding salty foods (which cause fluid retention), avoiding caffeine, chocolate, and alcohol, and taking an anti inflammatory medication like ibuprofen or naprosyn.
If none of these strategies work, then getting in to see your doctor
is advised. They will perform a thorough examination of the breasts to make sure there is nothing else going on.