I had a breast lump almost a year ago.got it checked, the ultrasound didn't show anything.could it be breast cancer? i turned 17 this year

Unsure: At your age a breast cancer is unlikely but still possible. If the lump is still present you should see your doctor for re-evaluation.

Not likely: You should be preforming regular checks yourself and with your doctor. If you still feel something but the ultrasound doesn't show anything, your doctor will suggest further testing. If the ultrasound shows a cyst, that can be watched. If the ultrasound shows a solid entity, a biopsy might me needed. At 17, you chances of cancer are low.