Same thing used to happen to my friends and me years ago when taking ephedrine. Not total impotence but some difficulty. The stimulants stimulate your nervous system and the areas of your brain that need to relax to allow blood flow to your penis can’t settle down. Stimulants will also impede the blood flow to your penis. Worse for some than others. Thats the layman’s way of explaining it.
When off stimulants everything should return to normal like someone else posted