I used to put water in my DII and then I put in this Prestone Extended Life Coolant (Advance Auto Parts: Extended Life Antifreeze/Coolant (1 Gallon) by Prestone - Part AF2000)

About a week later it starts overheating without leaks or anything. Could this be caused by this coolant somehow?!

 

Very doubtful that the coolant caused this issue.
Running just straight water will ruin your water pump bearings and cause scale and lime buildup inside your cooling system.

 

Did you cut it 50/50 with distilled water, or use it full strength? If the latter then yeah.

 

Pure Anti-freeze cannot release heat, which is why it should be diluted 50% with distilled water. In hot climates it is often diluted 60% to improve cooling. If you use mineral water (Tap water) you risk electrolisys which may damage components and even create leaks. So the short answer is "yes" if you ran pure anti-freeze it can cause over heating.

 

Ok, so what do I have to do to stop the overheating and stop it from happening again?

 

You need to drain the system and fill with a 50/50 mix of coolant.
If you buy it premixed you wont have to worry about the mixture because it will be correct.
You will need 2 gals of premixed 50/50 engine coolant.

 

And throw in a bottle of Water Wetter or Purple Ice

Eric