when throwing a punch, how hard to you have to "grip" your fist/arm? (my arms/shoulders/hands are relaxed when not throwing a punch, though)

is it normal to get pains in these areas after a couple rounds of power hits on the bag?

http://i90.photobucket.com/albums/k249/deh707/armpain.jpg

or maybe i am flexing/tightening/gripping my muscles too hard when punching? (i heard it's bad to tighten your muscles too much; causing drained stamina..)

those areas in both of my arms have been sore for about 2 days so far.

Thats just your muscles being worked out on the bag, totally normal. I've never had forearm pains/soreness from punching tho. Make sure you are hitting the bag properly.

nah, doesnt' hurt when punching, only later that day, especially the next morning.

but my wrists/knuckles are fine. i used to always hurt my wrists on the bag 10 months ago before my friend (an amateur boxer) showed me the basic form.