^^^all on one string^^^

I hope i'm not asking too many questions but:

Whenever I play this above section really fast, and over and over again, my fretting arm begins to get really tired on the other side of my forearm (where your arm hair grows). Now i've read that you are supposed to play without tension and my question is: am I playing correctly if this fatigue is occurring? or should I be able to play fast and long without any tension or fatigue? Thanks again =).
Does the fatigue occur right when you're playing the lick, or after you play it for a while? Because if it's after you play it for a while, you're just building up your fast twitch muscle. Just pretending to play that lick, I can feel my forearm muscle tightening and loosening.
If it hurts the second you start... you have AIDs. And something else is wreong.