Hi there,
The reason it happens is because there are three major muscles involved in flexing your elbow and the positon of your hand affects which one of those muscles works the most.
The muscles are as follows: biceps brachii (isolated by hand facing up/preacher curl), brachioradiallis (isolated by hand facing the body/ hammer curls) and brachialis (isolated by hand facing down).
the first thing to do would be to stretch the biceps brachii to see if that helps. you can do this by streching out your arm behind you and placing your hand face up on a table (almost as if you were reaching back to recieve a baton in relay). In this postition step forward and bend knees at the same time, keeping your hand on the table, until you can feel a mild to moderate stretch. hold for 30-90 seconds. Do 3 of these, 3 times a day.
oh and stretch off your triceps as well.
see if that helps, if not getting better in 14 days or so. repost.
good luck
p.s.: here's a link to a pic of a biceps brachii stretch: Google Image Result for http://www.physioadvisor.com.au/assets/256/images/12977256(300x300).jpg