Extreme weight loss with huge calorie intake?

Hello helpers,

I’ve been eating a lot for the last couple years averaging 4000 calories a day I reckon. I eat double the amount most people do in order to try and gain weight and I can’t? I am literally trying to get fat so that I can support more weight. Recently I contracted an illness called glandular fever and as a result I lost 9kg over the past month, going from 83 to 74kg (5’11/6’). Today I have eaten a whole pizza, cereal, steak and fried rice (huge portion), 2 lamb chops, half a cake of tiramisu and various other snacks which I calculated to form about 4500 calories and I’m sure there’s protein overload. I need some sort of explanation as to why I’m losing weight!? Thanks.