2001 Ford Expedition Won't blow good heat

I just did a super clean on the vehicle myself and it had good heat during this procedure.I drained the super flush and added antifreeze at 50/50 mixture and once it was full I started the vehicle to let it warm up, the heat blowed warm so I took it for a drive, the longer drove, the cooler it got.I also changed the thermostat.I'm thinking of changing the waterpump, it doesn't seem like it circulates right.I was told that I might have a blowed head gasket or cracked head, no signs of leaking or loss of coolant, the oil is clean. Any ideas would be great, thanks Robert