Both breasts itch severally on the bottom and they are too sensitive to put any pressure it worries me of cancer .

Skin : You may have a skin infection such as yeast.. You should see your physician to get evaluated. Could be a skin infection, mastitis, or other issues. Not likely breast cancer at age 17, but of course it is possible.

Have 'em checked: If you mean itching behind the fold of the breast, it could be as simple as a yeast infection. At your age, breast cancer is unlikely but why worry? Go see your family doc or pcp. If it is a yeast infection, you may need a topical antifungal cream. Once better, you need to keep that area dry. Best regards.