My car is screwed up, HELP!?

I drive a 2002 Pontiac Montana, and on my way home from work and I noticed that my cars speed started to go down, but it would steadily stay at 40 mph, then I come to a stop and I pressed on the gas but it wouldn't go past 20 mph. So I pulled into a McDonald's and let it rest for about 5 minutes, then I started on my way back home and I noticed it took a good 7 or 8 seconds( Like your counting Mississippi's}. My "Hot Coolant Temp" gage was on so I was a little worried, but I got home safely but still with a very, very screwed up car.