Need help, overheat problems after reflash?

ok so i drove my car today for the first time since the reflash, and I let it idle for like 15 min then i drove it like a mile down the road and the overheat light came on. I stopped for like 15 min then continued to drive it home which was like 4 miles away. As i started driving again it came on again but i decided to continue to the gas station a little ways ahead before stopping but before i got there the light went back off, then back on, then back off, then as i stopped and idled for a min it was back on. I let it cool down then continued to drive to the next gas station, and it did the same thing, on off on off on off, we decided to try turning the heat on to see and my heat isnt blowing hot air just normal air but my a/c works. I stopped at that last gas station and i got gas and let it cool down again i had like at least 2 miles to go to home and decided to leave my heat on full blast the rest of the way and as soon as i pulled out the overheat light came back on but went immediately off. it stayed off the rest of the way until i parked at my house and was idling and it came back on. And even after leaving my heat on full blast for 5 min in 90 degree weather it still was not blowing hot air. Now i need some suggestions or something, yes there is plenty of coolant so dont ask the stupid question please.