Seven x64 memory problem

[UtherIV]

Hello.
Recently, I left the PC on all night long with jDownloader (I use Windows 7 Ultimate x64). The thing is, when I started using it the following morning, it was quite slow... When I looked at the memory usage, it was at 2.8-2.9 GB (!) -whereas it was at 1.4-1.5 GB when I left it-. A 1.2+ GB usage increase!?
Why did the memory filled all by itself? What would cause this?
As this was quite strange, I checked at the Processes tab in the Task Manager, but there was no process that would consume like a (1) GB of RAM, just the same ones that were open the night before.
I'm no noob at using Windows, but this bothers me a lot.
Thanks to anyone that could give me a hand / answer =D
Bye

System Specs (if relevant):
ASUS P7P55D LE - Updated BIOS
Intel Core i5 750 @3.2GHz
OCZ DDR3 4GB 1600MHz @1608MHz 7-7-7-21
XFX Radeon 5850 1GB -Stock-

PS: According to the Perfomance Monitor, there is 514MB of RAM reserved for hardware, leaving 3.5 GB to "everything else".

 

[karlsnooks]

Hello IV and welcome to SevenForums,

And the problem is?

Personally, I never pay any attention to memory usage, unless I've got a problem.

 

[UtherIV]

Edited, now it should be understandable
Neither do I, but as the PC turned somehow slow, I was thrilled about that.

Thanks for the quick reply