Why has my computer all a sudden acting so slow?

[Love Vixen ✗♡✗♡]

I was one day on my computer after just turning it on and everything was fine. I went onto Team Fortress 2 and the fps on there was fine too, on the usual fps I would get. My cousin on Skype called me so I pressed the start-menu button to exit fullscreen mode and answer it. I later got an achievement on TF2 called "Trust Your Feelings." about 3 minutes after getting that achievement, I got bored so I thought I'd continue working on this project for Minecraft that I would be setting up for download once its finished. After loading up the world I got up some songs to play on Youtube then when I went to switch back onto the Minecraft window, I realised how I was all of a sudden really lagging. I tried Closing then reopening Minecraft and Google Chrome but no difference. I tried ending the Skype call but then when I got recalled again, I try to spam the answer call since it wasn't answering the first few clicks, it then answered but the incoming call sound was still playing for about 5 seconds after it had answered. I then tried turning the computer off then back on but it still lagged like hell and it is still being like this even now. Does anyone know how to fix this for me please?

Incase you need my PC info:

AMD Radeon HD 6450 Graphics card

AMD A10-5800K APU with Radeon(tm) HD Graphics 3.80 GHz

[kot]

прапраап

[Jonno.exe]

Too many games and application running. Just keep it to a bare minimum if possible or else you are gonna stuff up your computer

[derek]

Pointless to have a GPU with an APU
Just get an APU and get ♥♥♥♥♥♥♥♥ of fast memory.

[Love Vixen ✗♡✗♡]

Originally posted by Agent.J:

Too many games and application running. Just keep it to a bare minimum if possible or else you are gonna stuff up your computer

Yeah but I only use one game running at a time and its not just the games that lag, its the whole computer.