http://www.linuxatemyram.com/

The link @TheNumb provided explains what’s going on here. The Linux kernel will use any available memory for disk caching, unless it’s required by a running program. If needed by a program, the memory will be freed up. From the site Linux Ate My RAM:

Why does top and free say all my ram is used if it isn’t?

This is just a misunderstanding of terms. Both you and Linux agree that memory taken by applications is “used”, while memory that isn’t used for anything is “free”.

You would call that “free”, but Linux calls it “used”.

Memory that is	You’d call it	Linux calls it
taken by applications	Used	Used
available for applications, and used for something	Free	Used
not used for anything	Free	Free

This “something” is what top and free calls “buffers” and “cached”. Since your and Linux’s terminology differs, you think you are low on ram when you’re not.