Whenever you do bi's don't hyper extend, only let your arm go down to a certain point. If you'll notice, your bi relaxes once you let your arm go all the way down, you don't want that, and in order to keep constant tension on the muscle you stop right before full extension down, and stop at peak contraction on the up, and no further than that. Your tendons hurt for one of two reasons, too heavy of weight, and/or you actually aren't pulling the weight up with your biceps, you are actually using the tendons (which could tie in with too heavy weight). Not trying to discount you in any way bro from my comments, but I've had the same as you back in the day and thats what my problem was. They don't call me Cannons for nuthin bro!! LOL!!