Laggy MacBook Pro 2012

Hey guys.

I've got the MacBook Pro 2012 13-inch 4gb RAM since fall 2012.

Until this last week, I've been incredibely satisfied with it.

Then, suddently, one day, it started to lag. Like seriously lagging. My trackpad was responding late. ( I would move the cursor, but it would freeze and the suddently appear somewhere on the screen.) The spinning beachball appeared, wherever i clicked, and my applications would open like 30 seconds after i clicked them. I searched on the internet for what i should do, and after reccomendations, i installed OnyX. I used the automation setting and it cleaned everything up. After that, everything was working fine. The next day, I started my Mac, and the same problem occured. Everything was responding slow and late. I tried a night with sleeping mode on and a night where i turned my Mac off. Both with the same result; lag. Only after i clean the system with OnyX, ( which takes like 30 minutes), the system runs smooth.

So my question is, what can i do? I can't believe this is normal.

I haven't downloaded anything illegal and I am running Avira Antivir, which doesn't find any virus.

Please answer and help me :-)