Health-related question [read details]?

Recently I noticed that I can’t eat the same amount of food I could eat before I got sick (I found out I had mono, about a couple of months ago)
I eat less than I used to (I’m alright now, though) and each time I eat just a little more, I get dizzy, want to throw up, and have to lie down for a while.
Could all this be something mono left me? As a goodbye gift or something?

I also have that thing, if I stand up too fast I can’t see anything for a few seconds.. and when I’m nervous or too scared, same thing happens, together with a headache that’s like someone’s trying to open my skull with something