Running hotter

[azrps13]

It seems like the CA is running hotter than it did three days ago. Yesterday when i was on the freeway as I was climbing from 75 toward 80mph the temp needle was slowly climbing from the halfway point to the notch above that, then a little higher. Then as I decelarated back to 65 and below the temp went back to normal. Then I speeded up again and the temp followed. Got home and checked the radiator, Full. Im not sure if its just the Phoenix heat because this never happened before. The last thing I did before this happened was change the oil. Would the type of oil I used make this happen? It was 10-40 synthetic. Could i have just recently lost my thermostat. Is it because i have a SMIC or what , cause its really worrieing me. I dont want to blow a head gasket. Please help me!:(