I got a 92 Civic LX with 185,000 miles on it. My issue is that my car started overheating today, it was about 102 degrees outside & I was driving on the highway doing 70. I look at my temp gauge occasionally on a daily everytime I drive..the temp needle was in the middle last time I looked...15 minutes later(of driving on the highway) the needle was almost hitting the high mark. When I got home it was still on high..I opened the hood & looked under the car immediately..no leaks..no stream..no bubbles from the coolant reservoir or any other areas of the engine.
I even did the coolant protectant test with that $1 gauge with the 5 little color ***** you can get from Autozone...Gauge read protection -20 degrees for the coolant.

BTW...there was no white smoke from the tailpipe(thank god!)

2 months ago I flushed my coolant system(with a water hose) because I was changing my timing belt & putting in a new Water Pump and haven't had any Overheating problems till today.
BUT.....Weeks before that I had a coolant leak with my old water pump and put some leak sealant in the coolant system hoping it would stop the leak(thought the leak was from the head gasket originally..but no).

I looked on this site on what it could be:

It's not my radiator fan because I could hear it when I got home today.

It could be my Thermostat....maybe

Could it be the Leak Sealant crap I put in there when I had that leak clogging something? It never completely flushed out..even when I ran the water hose through the system for 25 minutes back then.

Any advice will do...thank you in advance!

P.S: Coolant level is topped off.