Running low on memory in Windows 8.1

There are times when I have some programs and games opened, then suddenly getting a notification that I am running low on memory...

When I look at my memory usage it is around 45-50% used, so I am perplexed as to why I am running low on memory...

My memory is 2 sticks of 4gb G.Skill Sniper 1600mhz for a total of 8gigs on dual channel...

Any help would be great! Thanks!

check for programs running in the back ground & check your page file