Feeling naudeous during flight

Hi, Yesterday I went flying and about 20m after takeoff i felt like throwing up. It isn't first time that this has happened. It happens basically everytime i have a longer flight (over 1.5h) in unstable (thrrmal) conditions, but yesterday it was much sooner than usual.

Has anybody had this kind of problem when they began flying longer flights? Did you find any way to overcome this or did the problem just disappeared as the body adapted?

I should mention that i am new pilot, 2 months after obtainint SPL.