Cell phone batteries today are very much built with lithium ion. The reson is that lithium ion is a great way to keep a battery from being OVER-charged during the charging session, but unfortunately the tradeoff is that the batteries have very low heat resistance. With time, the batteries will lose effectiveness--the battery will get hotter quicker, hold less charge, and get much hotter to the touch.
If you got your phone just this last summer, then take the battery back to Verizon and get a warranty replacement on it. They shouldn't give you any grief over that.
Let me know if you have any follow-up questions, otherwise please make sure you click 'accept' in order to close the question so that I am credited for my answer(s).