Tendons are starting to in"flame" Tendonitis is, and, can get progressivly more tender.
Try warming up your elbows by doing this movement. stand up, put your hands up, palms to the sky, now lower them as low as is comfortable, now raise them again, do this pumping motion for a whole 30 seconds, not reps. Idea is to warm up the arms before you strech them, never stretch "cold" muscles. Now take your left arm and raise it, now lower your left hand so that you are touching the back of your neck, with your right hand pull your left elbow backwards, to your back, not side, until you feel a good strech deep into the ligaments and tendons and tricep. Hold for 30 seconds and then do the other side. Do not bend your neck, keep it fairly upright. A couple times on both sides 2-3 times a day and you will ward off, if not cure, the tendonitis before it gets worse. Doing this before and after you shoot will also get rid of most elbow problems.
I would also let the bow arm have a natural curve/bend to it helps to act as a shock dispenser.