Overheating

I have a 1999 Corolla LE (1.8L 4 cyl engine + 4 speed auto transmission) with around 125000 miles. Maintained regularly with all oil changes, all fluid changes, radiator coolant flushing, etc. Performed on schedule. The last radiator coolant flushing was done a few weeks back together with the 120000 mile maintenance schedule.

Today I drove around 20 miles on highway with AC on (outside temperature around 98F). Thought the AC was not as effective as usual. At the end of the drive when I glanced at the engine temperature gauge, I found that it was in RED range (I didn't look at the gauge while I was driving).

Am wondering if there is anything that I can check before I take the car to the mechanic. Would like to have some idea of what could be wrong so that I don't get misled.

Thank you & appreciate your time and help.