Are you familiar with where your diaphragm is, in relation to this pain?
My guess, and based on my own past experience is that adding the hills caused you to go over your ability to deliver oxygen/breathing hard and over worked your diaphragm. I had this issue early on riding with a club I joined some years back. In my own case it sort of felt like my liver and kidneys were messed up. It went away over time.
I am not attempting to discount serious possibilities here, but WebMD will almost always tell you that death is imminent, etc. and the folks here are smart to err towards caution just in case you are...