No period, ain't pregnant

Hi, I'm 21 years of age and about 4 months ago I came off the injection and it had regular periods since I've came off it, but on July I missed my period and havent been on since, took about 9 pregnancy tests and a doctor one and all negative, doctor said could be a phantom pregnancy caused by the coming off the injection or ectopic pregnancy (which is worrying me), anyone have any clues on what it could be or has anyone else had something like this?!