A home pregnancy test is about 99% accurate, when used after your expected period has not started. You said you took the last test 2 weeks ago, which means the first test you took was probably too early to be accurate. I am unsure when exactly your last menstrual period was, what date you took the test (when you are expecting your period now). If you want to confirm that you are not pregnant, you can take one more HPT now. If you are sure that the last HPT you took was after the date your expected period did not start...you can assume those results are 99% accurate.
I am concerned about your symptoms of having "extremely tender" breasts" and "strong cramps", and not because of pregnancy. I assume your employer allows for employees who are sick or injured to seek medical care, and your symptoms of pain for a few weeks indicate that you should see your doctor (your regular gynecologist, or your primary doctor, or go to a walk-in clinic) so they can determine the cause of your pain. (Pregnancy symptoms do not include "strong cramps", and since your period is past the 35 day-cycle, you need to also call your Gynecologist office and talk with the nurse to ask for his/her medical advice).