new hot water heater problem

Hello all. I recently replaced our hot water heater because the old one was leaking. Now, when showering if the toilet is flushed the shower gets very hot. If the sink is turned on the shower gets cold. We didn't have this problem with the old heater. It was a 38 gal and the new is 50 gal. Any idea what could be causing this? The old one was soldered all the way to the water heater and for the new one I used some flex pipe with shark bites on one end and it screws on the other. Not sure if that makes a difference or not. Any help would be appreciated. Thanks.