What Causes Delayed Period And Lower Abdominal Cramping While UPT Is Negative?

My period is 1 week late. I have taken 2 home pregnancy tests that have come back negative. And i know have been having cramping and my lower abdomenal area has been tender when I push on it. i went to my dr and he didn't do any testing. He beleives that the cause for not getting my period is due to the lining of the uterus being so thin. I wasn't too happy with this answer. I need to know if i could be pregnant and should go to anouther dr.