I started drinking Mate later in life, and definitely recall it sitting weirdly in my stomach at first. You might want to try drinking it in smaller concentrati

I started drinking Mate later in life, and definitely recall it sitting weirdly in my stomach at first. You might want to try drinking it in smaller concentrations? i.e. steeping for less time, with a smaller quantity of "tea". A random theory, but since the chemical profile of Mate is quite different from coffee, it might be your body going through selective withdrawal from coffee that's producing the "weird" effect.