Can a test for cancer in a breast lump be wrong? It was tested not cancer but it grew back now doctors are worried.

No perfect tests. There are no perfect tests, and unfortunately these include the ones that we rely upon to tell us if cancer is present or not. If there is doubt, or your clinical situation has changed, please don't delay in getting a confirmatory study or second opinion. Ask your doctor to candidly share his concerns and best direct you appropriately.
Unlikely. I assume that the test you had was a biopsy. If so, it would be very unusual for this result to be wrong. More likely, some benign tumors can grow; in that circumstance, it is safer to do a small, outpatient operation to remove that tissue.
Get 2nd opinion . I recommend you go to the american society of breast surgeons website to find a breast specialist who can review you case and make recommendations.
Grew Back? It is a difficult question to answer without facts, but unfortunately, a benign tissue may become malignant "years later." I am a cancer survivor and quoting my doctor who said 16 years ago, the cancer "may come back in 20 or 25 years." there were instances i felt the cancer is back, but regular check-ups so far showed otherwise. I hope and pray, your test will be negative for cancer.
Fibroadenoma. If you are 19 and the lump was benign it was probably a fibroadenoma. These grow back on occassion or you may have another condition (like fibrocystic or very rarely cancer) in the same area. Most likely a benign condition if you are 19, but best to have it checked out again.