Last nite i was driving and suddenly the add coolant light came on. I have an aftermarket temperature gauge and the temperature started to rise rapidly so i switched off the engine. When i opened the hood I could see steam coming out of the cap which is on the plastic tank(the thing which the AST replaces) the cap was loose so i tightened it and add coolant. After that car ran fine, any idea what might be wrong?

 

The stock AST might be warped, interfering with the proper seal of the cap, allowing both pressure to escape and the cap to loosen. Either way, you have a problem with the cap, the AST, or both, so I recommend you replace them with an aftermarket version. It may be just the cap but it never hurts to upgrade the AST.