My son who is almost 3 months now did and still does exactly what you're describing! Tenses his muscles and arches his back constantly! He does it most often while bottle feeding (nursing was a horrible experience for the both of us and it only lasted 2 and half weeks) and when he gets really frustrated or tired. With time, I've found tricks that help, but they never seem to last. You'll quickly realize if you haven't already that you constantly have to change your methods with babies... as soon as you think you have it figured out, they change!
Here are some things that have worked for me:
-letting him lay by himself... sometimes he just doesnt want to be bothered. he loves him gym and i often put him in there while i shower (usually right after his bottle when he's the happiest)
-giving him breaks while he's eating
-if he's frustrated as he's eating, i've put him on his change table to eat... since he's calmer there
-also, it sounds a little mean, but when he's so frustrated and tired and he tenses up, i hold him really tight and rock him while bouncing him, patting his back and shushing in his ear (anything to get him to calm down)
i don't know if you swaddle your little one, but that seems to help my son out alot.
good luck
