I am sorry I can't answer your question as a medic - on this part of the forum we are just people with breast cancer as I have, or people who are worried about it.. Your lump could be a symptom of your normal monthly menstrual cycle, a simple fibroadenoma, or a cyst, none of which are malignant, but you need to see a doctor and get it checked out. You don't mess with a lump in your breast! don't know if you have read Lee Child's books, but he has a character called Jack Reacher, an ex Army military policeman, and his favourite phrase was: "You don't mess with the Special services", and I believe you don't mess around with lumps in your breast You need to get it properly diagnosed, just for peace of mind. If it is a problem you can then go on to address it and get appropriate treatment.
Take care - and go see a doctor!
You could be having a fibroadenoma, which seems most likely as it alters in relation to your menstrual period. It could also be a cystic lesion. But for a confirmed diagnosis a clinical evaluation by a physician would be required. The further management would depend on the diagnosis. Have you noticed any discharge? Try some hot compressus and breast massage and OTC painkillers till you schedule an appointment with your physician. Also ask your physician if a mammogram is required. Goodluck.