If it is everything then there a plethora of places to start. Neighbors would be first in my list. Especially if they are on the same transformer or line. Not if they are on a different transformer or line. Next would be to check all of the connections in the panel. Not a good place to play unless your comfortable working around an invisible servant that wants to kill you. Some humor. It would be handy if you had a voltage meter that had a scale that you could measure the voltage. Can you borrow one. Needs to have a zero to 300 VAC scale. Radio Shack has some that are less than 100 bucks.
If it persists then try the utility. Now your heading into a mess. There are standards that they must provide. It is plus 10% to minus 7% of nominal. That means that the average plug could read any thing from 132 to
111.6 volts. The sad part of IEEE 519 is the next paragraph says except for short periods of time. No definition on this one. Is a day a short period of time compared to a year, you bet.If you do not want to borrow or buy a meter then it is probably time to call someone to come over and check it out. Plan on the problem going away when they arrive. It usually does, my experience anyway. This might need a recording meter left on the service for at least 24 hours, I would want a weeks worth of data.