What Causes Nausea And Sense Of Fullness After Immediately After Taking Food?

it will be lunch time and ill be hungry my stomach is making noise so ill get my lunch to eat. I will take around 4 or 5 bites then my next bite will make me feel sick or i feel like i just ate a big turkey dinner and feel completely stuffed. I will not get hungry for hours later and when i go to eat the same thing happens. I used to be the total opposite and loved food now nothing really appeals to me. i have lost 50 lbs because of it im not sure what is wrong