1998 Cadillac Seville Multi

I had my radiator and filter changed and my car over heats after driving for about 45minutes or more; but there is no leak of antifreeze from my vehicle; could it be my water pump that needs to be replaced? Something is causing the pressure to build up and it seems like it is building up tremendously in the container in which I pour the antifreeze. After sitting and letting the vehicle cool down; things return back to normal and I don't cut on the AC due to fear of the problem returning. Also, the engine fan comes on autmatically as soon as I start up my vehicle; I thought that only happened when engine is hot, but it's early in the morning and I am just beginning my driving day. What could it be?

Thank you for your time and consideration.