How many Ram sticks does your Pc have? 2? 3?
I would say either one of your Ram sticks is bad or faulty.
Check if all of your RAM is placed and locked in correctly. While doing this also check if there are any bent pins
Edited: Download CPU-Z and see if it displays correctly in there also check in your BIOS to see if it displays the correct amount.
You may have to restart your Pc a few times for it to refresh.