Empty pregnancy!

So... I've been having pregnancy symptoms, abnormal periods etc for a few months now... last normal period almost 14 weeks ago. I've had negative tests... urine and blood, but am growing a pregnancy belly and boobs have grown 3 sizes. Even started lactating. I FEEL pregnant. I had an ultrasound done which shows no evidence of any fetus but a 2.2 cm thick endometrium... it seems... my body thinks and is acting like I'm pregnant... except I'm not. Has anyone had anything similar? Any thoughts? Perhaps I WAS pregnant briefly, didnt develop and my body doesn't realize it? If so... when on earth will my body clue in?!