I have a breast lump. Doctor didn't find anything. Ultrasound showed nothing aswell. Its painful and I feel as if it has decreased in size. I'm 17.cancer?

Normal breast tissue. Women of your age have very high levels of estrogen and progesterone -- female hormones. These can make your breast tissue swell and shrink, especially with your cycle. Your ultrasound likely DID show something -- normal breast tissue. Cancer is almost NEVER painful, and NEVER gets smaller. This makes cancer very unlikely. Examine your breasts monthly AFTER your period, if changes, see Dr.!
More Info needed. What makes you say you have a breast lump? Do you feel a lump? but the doctor doesn't? Ultrasound doesn't show it?? Are you just having pain? There a many reasons for breast pain. Caffeine being the most common. See a breast surgeon if you still have concerns.