cooling system leak

My '90 Corolla has a mysterious leak. One and half months ago, i just installed a new aftermarket rad bought it from the web site. New OEM rad cap 7 new coolant. Just 3 days ago, the temp needle gauge went up to the middle of the temp range gauge, usually temp needle rested at about 1/3 of the temp range. Immediately, I opened the cap and found Ilost a quart of coolant including the water in the overflow tank. I noted somewhere at the i/3 location of the rad leaking coolant after engine shut down. Today i drove and checed and found no leak. Water in therad is full. Any idea to inpoint the leak? Any response is appreciated.