Please don't panic,this could be one of many benign lumps, such as cysts or a fibroadenomas which are very common.
Take comfort in knowing that a large lump that appeared only 2 weeks ago,is HIGHLY unlikely to be cancer,especially at your young age.
Not all palpable lumps will appear on an ultrasound or mammogram. Each test has it's limitations and what it can pick up.For example an ultrasound is better at differentiating between a mass that is solid or fluid filled.A solid mass may or may not show up on Ultrasound...When this happens, it is usually recommended that the lump be biopsied to see what it is exactly.
A fine needle aspiration(FNA) is a quick simple procedure.Fluid will come out if it's a cyst.If not, tissue in the needle can be examined under the microscope for a definite diagnosis.
Hoping that all will turn up just fine for you...
Best wishes and good luck!
That just made me feel a million times better! Thank you!! :)