1997 Dodge Avenger Antifreeze

I own a 97 dodge avenger and I changed the antifreeze by flushing out the system ( I did this in my own garage no flushing machine just did it the old fashion way) anyways one day I was warming my car up one day and about 15 mins later I walked outside to leave for work and noticed antifreeze leaking from underneath the car. Was leaking pretty good. I turned the engine off and checked to see where it was leaking from. My first thought was the water pump but my car didn't overheat temp gauge stayed on normal. So I keeped on looking. Couldn't find anything that I could see where it was leaking. I checked the coolant tank and it was low so I added some to the fill line and drove it to work. A few days later I checked it again and it was almost to the top (the coolant tank) so I dumped alittle out but not to much till it was on the fill line. I checked it again it was almost empty. I'm scratching my head can't figure out why it could have done this. My? Is dobubthink I used the right amount of antifreeze after flushing it? Not enough? I used that consentrate antifreeze so I added water to it. I put 75% antifreeze 25% water so did I do it right or do I add more or drain alittle? And for the leaking problem what do I look for? I checked the coolant tank no cracks or doesn't show any signs of leaks. Do I have a big problem or a little one? It hasn't leaked since anymore that was the first time it has done this and hasn't leaked since. ( Was leaking on passenger side of car) has the 2.5 liter v6 engine.