Hi..
Myself Dr Shivpal Saini, (Surgical Oncologist)
At the age of 17years, breast cancer is a little unlikely diagnosis.
The slight difference in breast size is normal and it is present in approximately half of women.
Breast size difference, tenderness and tightness is more prominent during menstruation.
If you feeling a discrete lump in breast still it may be fibroadenoma as it is more common in this age group.
Next Steps
You should undergo a dedicated ultrasound of bilateral breast.
which will help you to be sure that nothing is abnormal.
Further if you want any help you can consult connecting via audio or Video call with ultrasound report.
I hope this will help you.
Health Tips
A sudden onset lump in breast is a little worrisome.