If you’ve been having muscle fatigue even if you stretch before working out at the gym, it might be due to lack of a proper warm up session before the workout proper.
Basically this means that:
cold muscles + stretching = pain!!
You need to properly warm them up for the muscles to be more flexible. Imagine comparing it to rubber bands, if you chill a rubber band and then stretch the rubber right after it comes out of the cooler the tendency is it will reach its limit or break where as if you warm it up it becomes much stronger and will stretch further.
Such is the same with our muscles! So remember to do a warm up first and then stretch to reduce risk of injuries.