Welcome to HCM
We understand your concerns
The problem of fullness you are mentioning could definitely be of acidity or loss of appetite. Don't worry.
Some heavy food or junk food takes time to digest and the left over portion is starting to decay inside the colon
and this emits some bio-gases. The acid present in your stomach and these gases combine to produce acidity which manifests as fullness of stomach.
Combining this, if you are lazy, without doing any physical activities, it is also possible that even if you do not eat for some time, you may not feel hungry at all. Therefore, start exercising and have healthy and fiber rich food.
Hope this clears your doubts. Available for further clarifications.