To “stunt” is to show off; “catching stunts” involves a car, in this case a BMW 745. 50 plays on the car reference in the previous line by stating that the girl drives him crazy. He is so infatuated by the girl that he finds himself trying to impress her by “catching stunts” in his 745. The girl is driving 50 crazy, as 50 is literally driving his 745 in an attempt to stunt.
Sometimes “shorty” means “little one”, sometimes it means “girl” (here it means girl).