Heating the area is only going to make it worse. When you feel pain, you need to be icing it down. Tendonitis is actually an inflammation of the tendon right? You don't want to heat that up as it will just draw fluid to the area and thus delay healing.
Ice.
Ice.
While watching tv, ice that area for 15 mins or so. I mean, you are already just laying there.
Next is massage. if you prod around the area with your thumb, you eventually find THE point where it hurts the worse. Oh, you will know. You will be touchng areas that hurt and suddenly you will press a point that is *GASP* the one. That is the area you want to massage. YOu want to get some cross fiber massage and friction in there to help break up microfibrin and potentially forming scar tissue. You just press down on that area and very deeply and steadly rock your thumb back and forth, back and forth. It should look like you are barely moving your thumb. Not a big movement at all. Just deep and steady. It only takes 30 seconds each time. You can do this on the toilet. Sitting at redlights. In the shower. Whereever. As many times as you think of it, do it. Any extra times you have, do 30 seconds for the surrounding area too. Most of the time the majorly painful area is going to be closer to the bone rather than in the middle.