Pontiac 2002 Grand Am Running Hot During City Driving

My car had been running hot for about a week, being the worst when I tried to use the heater. At that point it had been getting up to 230, at which point I turned the heater off and it would quickly reduce back down to its normal 200. Two days ago it did this, but with just the fan on.

Yesterday it reached 230 just sitting idle for a couple minutes. I took it on the highway because I had to use it to get to my college class, and it stayed steady at 205 while I was on the highway, but as soon as I started driving in the city it heated up to 220-230, same on my way home.

I went to the store and bought it coolant, which it needed and my sensor light hadn't come on due to Pontiac's known electrical issues. I drove it down to class again today and before leaving town it had gotten up to 220 again, going down to 200 while on the highway, and when I reached the college town it went back up to 220 before the fans kicked in and brought it down to 205.

Anyone know if this is just the car refilling itself with coolant or if I should be worried?