it normally is in your form if your sting is contacting your forearm; your bow arm is not in the correct position.
if you shoot a longbow or recurve, your brace may be too low causing the string to "whip"...a compound...limbs adjusted for too low of poundage for the rated bow rate and/or too long of a draw length.
you may possibly be "overbowed" causing you to strain to draw the bow. that in turn, can cause you to have your arm in the wrong position.
you may be "underbowed" enabling you to draw TOO FAR, causing your arm to be in the wrong position.
try to anchor to the same spot each and every time (normally, the forefinger of the drawing hand to the corner of the mouth)
follow thru (lack thereof, rather) and collapsing of the bow arm on release may also cause string contact.
also try leaning/tilting your head forward just a little when at full draw and anchor.
believe it or not, the primary purpose of a brace (armguard) is to keep loose clothing from snagging the string as it passes the arm after release.
most likely, though, it is in your form...readjust body positions and anchor until you find the correct position for your application.
