Little question concerning my PC and FPS

So I was playing an online game called Dota 2. When I found a match I noticed immediately that I had major FPS problems.
Well, at first I thought it was just connection problems but when I payed attention I noticed it wasn't the regular "high ping" FPS problems. After winning the match I wanted to try out other games if they had the same problems. First Team Fortress 2 to see what my ping was as I can't see it in Dota 2. My ping was a solid 15-20 but my FPS were about 20-30, which would otherwise be around the 70.

My next step was to play a single-player game; I picked Shogun 2. When I did some custom battles I noticed that the animations were okay, but when I moved the camera he wouldn't go smooth (sorry don't know how to describe it otherwise). So I turned down everything to low and small size army. Subsequently I did a custom battle with 1 unit on both sides; this went smooth, without problems.

Then I went on to Skyrim. When I launched Skyrim everything went as it always went; the loading times weren't longer. However when I was in the world my FPS was about the 10 (which I checked with Xfire). Normally it would go smooth, however I don't know what my usual FPS is.

Last I launched Mafia 2 which had the same results as Skyrim.

I did and do not understand what had changed. And yes I said did, cause when I rebooted my PC, as last effort before I would go to bed, and launched Skyrim the game played as it has always played; smooth. (Same goes for Dota 2)

So my question is: What could have happened? And why did it return to normal when I just rebooted my PC?

I hope one of you can answer these question.

It sounds as though some sort of process was trying to take over. Maybe your AV software decided to do a scan? Usually an unexplained system-wide slowdown is due to something like that.

If you're really stumped, you could always go into task manager the next time you see this behavior, and check what's taking your processor's attention.

Sorry, didn't mention that.

I did go to task manager and everything was normal (well, how it's usual). No heavy process running.

Also no AV scan did check on that too.

I may be wrong but I thought it wouldn't be my processor cause the load times for the setup for the game were normal.

Seems more like your RAM needed to be flushed via a reboot. Sometimes graphics card drivers also need a clean boot. Everyone gets this problem once in a while. Not too serious.... if it occurs more frequently however, then come back and report that!