You all had your human performance lessons before taking flight lessons?
It is not uncommon to have quite a learning edge before you understand your own physiology. Everybody is a bit different and it takes time to understand your own body(fluids and when they will greet you again upways).
Try to figure out under what circumstances you do not feel well. Is is nervous, is it hungry or fed up, better coffee or tea b4 T/O, one of my students cured it by one liter of Coke (which usually gave a biotechnical time limit for the first flights), for me it was an allergy causing in-ear irritation and found by surprise by the flight doctor ... So, relax and take your time to find your personal reason.