What Causes Severe Abdominal Distention?

I am 68 years young , 5 10 tall and weigh 150lbs. I stopped smoking 6 month ago and have not changed my eating habits. For 3 weeks now I have an enlarged stomach, bloaded very uncomfortable. I asked my MD for an ultrasound. The results where negative. How can my stomach blow up over night? Thank you, Juliane Krebs e-mail: YYYY@YYYY