I'm worried about my car what could be the problem?

Today I was on my way home from work. I went up a steep hill then made my way up another steep hill. When I stopped at the stop sign at the bottom of the hills I noticed smoke coming from underneath the hood. I headed on home and when I got home I popped the hood and antifreeze was everywhere around the reservoir. The day before I had put antifreeze in and may have overfilled it. I took it straight to a local Midas and had it checked. They hooked a pressure test up to it and they said it was showing a tiny leak. They hoisted the car up and used a light and looked around and noticed a small leak coming from the timing belt cover. They said it was just a small seepage nothing to worry about. They then put the car back down and let it run for about 20 minutes with the heater running. The car didn't leak and it ran fine. I'm worried about a blown head gasket but the car isn't showing any signs of that. I have checked the oil many times but it isn't milky or anything. The car has great power still and isn't holding back at all. There's no steam coming from the tailpipe but there's water dripping from the tailpipe but it is clear as can be. The mechanic said that's normal due to condensation. The water pump is less than two years old. I left the mechanic and drove it up more steep hills. I checked it afterwards and had no leaks whatsoever. I also started the car and let it run for a few seconds and took the radiator cap off to check for air bubbles. None. The mechanic said in his professional opinion the head gasket is fine but you never know. I lose no power going up hills I do get a bit of a spark knock though. The spark plugs and wires are also new. I'm not sure what could have caused the antifreeze to go everywhere that one time. I have two ideas. 1. I overfilled the reservoir and when the car heated up it caused the reservoir to overflow or 2. air was trapped in the cooling system and it pushed the antifreeze out of the reservoir. Any help would be great.