That sounds very similar to what my heart does. It happens every now and then and freaks me out, but it doesn't last long and I havent been to the doctors for it, so I cant tell you what it would be. =[
Aww poor guy! I hope you find out what is causing it! I reccomend not researching stuff like that online, because there are so many things that could have similar symtoms. You will just end up freaking yourself out more, thinking it is something worse than it is!