There could be many reasons for this;
A new program is eating up memory
Malware is eating up memory.
Some programs, if left running can cause memory leaks.
If you get time to play around, AND ARE CAREFUL, type msconfig into the run box, tick the "Startup" tab, and google what's running in there to see if you really need it.
The Bleepingcomputer database
Also, download and run Malwarebytes
to make sure you don't have a virus.