Ok why won't my body let me do it anymore?

Ok so I've gained some poundage and my face looks chubby and I'm feeling gross. In my 20s through like 30/31 I used to live off 1000 to 1200 calories a day maybe less and would exercise like a maniac to stay very slim. I recently tried doing that again and i swear I've had terrible heart burn for the past day and I've just felt sick. My body won't let me do it anymore at 33 years old.