What can be causing my overheating issue?

Hi, I have a 2002 Mazda 626 LX with a 2.5L V6 that I've owned for 9 months and I have been having overheating problems (assumed by my in dash gauge) for the last 5-6 days. I've checked all fluids for the last 5 days 2-3 times a day and all are fine. With the exception of adding about 1/4-1/2 a cup of antifreeze every week for the length I've had the vehicle. Both fans work and run. The main driver side fan kicks on when i hit normal temp and the second fan (i normally on see work with AC running) comes on when it reaches 'H' on my gauge (with AC off). I just let the car idle from cold for 19min with hood closed and had no issues. At that point I took it out for a drive, 45sec into driving it was at 'H' on the gauge and stayed there for the 5min duration i drove for. Got back home and parked the car but left it idling with hood open. After 4min30sec the temp had dropped to roughly between normal and hot on the gauge and during that time it sounded like i had a lot of air blowing out of my overflow hose on the plastic coolant reservoir.  My question is: What could be causing this issue and what can I do to fix it?