If you had an Ultrasound I'm sure there was some indication in the report from the Radiologist as to the nature of the lump you describe. It certainly sounds like it may be a cyst which is quite common among nearly half of all women. These cysts do cause some discomfort and are related to the hormone levels in the body. You need to obtain a copy of the report if you have another Ultrasound and ask for a detailed explanation of the report from your Dr. I don't see any relation between what you describe and breast cancer.
It is very unlikely that the lump you are feeling is cancer. Many young women have cysts or fibroadenomas in their breasts that increase in size and become tender prior to their period. It will likely get smaller after your period. It may or not go away. Cysts can vary greatly in size and do not need to be treated in most cases unless they cause persistent discomfort. For your own peace of mind, keep the appointment that your mom made to see a doctor who is the best source of information concerning this lump.It is very wise of you and I congratulate, for keeping vigilant about any changes in your breast.Take care and God Bless.