Overheating mustang 02 V6

I sat in my car for 20 minutes, while it was idling
with Ac on, Ac started to feel warmish, service
engine soon light came on, temp gage was close
to the red. Turned it off immediately, waited 30
mins, drove it 1.5 miles to get some water to put in
it. By this time, engine had cooled to half, which is
where my car stays. Anyway, removed cap, went to
add some water, only took a quarter of a gallon.
Overflow was half full. So. I drove it 15 miles home
& it never got hot again, stayed at half. Pulled
thermostat, dropped it in boiling water, it opened.
No leaking from anything. Any help is appreciated.
Just to diagnose is $80, then the repair bill, just
can't afford it. Help!!!