truck randomly over heating ?

bought a 1999 ford f150 v8 5.4 engine.112,xxx previous owner said they had water pump replaced and a flush done on the truck. I drove the truck on Wednesday 45 mins to get it home (everything was fine) yesterday drove it down the road 5 mins and it started to get hot. I stopped let it cool down and checked it out. I thought I saw what looked like oil in the coolant (I could be wrong) but there was no coolant in the oil. after letting it sit and cool I took the truck home (same distance) and it didn't even get close to over heating. could this be the head gasket or something else in the cooling system ?

If they just did that work, might have air stilled trapped in the system.