HELP!!!! I am loosing coolant. There are no leaks in the system and my engine oil looks great. Why am I loosing coolant and where is it going??? Please Help.

Hi Guys, My 01 Sentra GXE jumps from normal to Hot suddenly... After driving it for a while, it again returns to normal. When I pulled over and checked, the car did not feel too hot, but the coolant overflow tank was completely full over the max level. While driving, whenever I switch on the...