I'm a heterosexual male and need a physical. the last exam i got an erection when the male doctor examined my genitals. is that common?

Any time you have: tactile stimulation of the sexual organ, arousal is possible, even if there is no actual interest or attraction. What you want to look for is a pattern - if there is a pattern that appears more than once, consistently, then conclusions can be drawn. But anything can happen once in your life for no apparent reasons. Pay close attention to the signs if it happens again.