This is a really good question, because it points to exactly the difficulty with the self breast examination. Especially in younger women, it is very common for people to find small ropy or lumpy feeling masses in their breast, and these can be totally normal and harmless.
For example, many women have something called fibrocystic changes, which are stringy, ropy consistency lumps in the breasts that wax and wane in prominence. Similar, young women often have rubbery lumps, particularly, in the outer half of the breast, called fibroadenomas that are totally benign and normal.
A more concerning lump would be something that was hard and fixed in the breast or that grew steadily in size (although benign fibrocystic changes may sometimes be more prominent along with the menstrual cycle).
The bottom line, therefore, is that if you go ahead and perform a self breast exam and find something, you shouldn't immediately start to worry
. However, what you should do is get in to see your primary care doctor
or your OB GYN doctor. They will be able to perform a breast exam to see if they can feel what you noticed. Based on what they find, they will either tell you that it is a normal finding (very likely) or they will recommend further testing.