For a detailed explanation of meat toughness, see What makes a moist steak (or roast)?
As for your specific case: I also don't know about American cut names, so there are two possibilities. Either you chose a collagen-rich meat, or you chose a cut low in collagen.
Assuming that you want fry steak in the pan and not have it become tough, you should choose meat which is low in collagen. If you didn't, you have to change the cut (or not cook it like a steak, as Rorschach120 advises).
If you have tender muscle and it becomes tough, you have overcooked it. "A few minutes" sounds too much for such thin strips. They are cooked until they first show color (browned, not just grey). If they get tough until they show color, you need to increase the temperature, and possibly ditch the nonstick pan.
Another option is to fry it as thicker steaks or oven-roast it. Then you can use a meat thermometer and achieve your preferred level of doneness. Cut afterwards and sear or torch the newly exposed surface.