Hi I_miss_my_kitty,

No, lumps are not always cancerous. Is it possible? Sure, but try not to worry. (I know, how can you not) Lumps are very common. Good for you for telling your mom!

Here's a paragraph and a link to an article. I hope it eases your mind a bit.

Breast cancer is extremely rare in 18 year olds so your doctor was correct in thinking it would not be cancer. The most common “lumps” that are found in adolescent’s or young adult’s breasts are cysts (small collections for fluid) and fibroadenomas (extra normal breast glands and fibrous tissue). Both are benign (not cancerous) but cause women to worry.

http://www.teengrowth.com/index.cfm?...ID_Advice=7946