We had a problem with the pipe leaking and had to have a plumber replace it, today I came home and it has two valves that look like faucets, one at the top and one at the bottom.. The top one was spewing when I came through the door.. It quit on it's own but I don't know if it will do it again.. I don't know how old the waterheater is, can't find a date.. could it possibly be going out? If it's too much pressure how did it change on it's own and how do we change it or fix it?
Although I can't see your water heater, I infer that what you saw spewing water might have been its temperature/pressure relief valve. It could act as you say if you were having thermal expansion issues. There are several Tanklets on that, as well as a simple test for it that you make with a $10 water pressure gauge. Go to Tanklets by way of the link at the top of The Tank topics page and look for Temperature/pressure Relief Valve Issues.
If you have thermal expansion, you'll need an expansion tank.
