If you mean that you get backspin on the ball...that is the expected result for all fastball releases from all overhand pitchers.
However, if you mean that you get the same spin axis angle on your fastball as someone with a much higher functional arm-slot than you have, well...to be blunt I wouldn't believe that without clear, unretouched video evidence.
The reason is: If you release your FB normally, that is, with your palm facing forward at whatever your arm angle happens to be at your release point--then your arm-slot will determine the angle of the spin axis on your fastball.
If you want some examples, my son is a pure sidearm RHP. His 4-seam FB has backspin and it's spin axis is exactly perpendicular to the plane of the ground. I've seen plenty of high-speed video of it, and that's what it is.
On the other hand, somebody like Trevor Hoffman--with his 11+ o'clock arm-slot at release--can also throw a 4-seam FB with backspin, but its spin axis is nearly parallel with the plane of the ground.
So, I suppose there is some remote possibility that you don't release your 4-seam FB normally, but if that's true then...whatever it is you are doing, I wouldn't call that a standard fastball.
As future-K suggested, you can change the spin direction and spin axis angle of your pitches with various degrees of pronation or supination at release--however, pitches generated in that way are not simple fastballs.