can being really full cause sleepyness?!

There isn't anything wrong with you, this is totally normal that you feel sleepy after eating. This happens to everyone, have you talked about it with someone else? If you ask someone, they will tell you they also feel like sleeping after their meals. This happens because when you eat all the blood float right to your stomach in order to digest the food you have eaten, thus you have no will and mood for anything else, you just feel lazy and doing nothing. This is why it is healthy to have a rest after heavy meals so that your stomach can do its job properly. You just don’t worry about your sleepiness, it lasts only until you have digested the food you have eaten.