Pea sized oddly shaped 'lump' at side of breast inbetween ribs moves when lift arm, going doctors but want to prepare myself before what could it be?

Few options: At 18 years old, the chance of breast cancer is very low but not impossible. The more likely things would be a fibroadenoma or breast cyst. These are both benign. Also, it may not be part of your breast at all. It could be a lymph node or a cyst in the tissue that covers your ribs. Any time you find an abnormality, it is stressful. Hopefully your doctor can put your mind at ease.