How long has this been going on? Are you sure you don't have a small leak somewhere? I actually had this happen in a house that I once lived in. The water pressure was off a little for about 3 months. The house had a small whole-house water filter in the crawlspace that I changed every 3 months. I finally realized that the last time I changed the filter, I neglected to turn the dial on the filter system all the way open to allow water flow back to the pipes. When I changed the filter again, I made sure to open the valve fully to allow for full water pressure. Problem solved!

Also, if you recently opened or closed your main water shut-off valve, that may be the issue. If the valve is bad, it might not open all the way.