Possible blown head gasket

Truck got hot shut down no oil. Put oil in it got home changed the thermostat. Had a 195 I think put a 160 thermostat in it. When replacing it I did not clean it good enough and water was coming out of the housing bad. Cleaned it up and no more water came out since. Now when I drive it, it will get hot after about fifteen to twenty minutes and after that I will begin to lose power. Also, the radiator cap will begin to squeal hearing the pressure come out sounds like. Had only happened once this is the day after I replaced it and I was coming home from work. Did not have coolant used water and during all this the oil cap is milky and dipstick oil seems fine just a little bit of water on it (tiny bit). I can also look down the oil cap and see some water on rocker arms. No leaks come out of anywhere on this truck.