Arms hurt when doing planks!? (sport related)?

Yeah if you are more "heavily built" up top (broad and heavy shoulders), it causes the weight to be distributed more to that area. Your shoulders are also not made to stand up to serious strain. When training always know your limits, if you start to feel severe pain, back off and work your way up. It's better to take our time and develop more slowly than to hurt yourself by being in a hurry! Hope this helped! :-)