1999 Pontiac Grand Am SE1 Coolant issue

I have a 1999 Grand Am SE1 it has approximately 122K miles and is a 3.4liter V6 engine.

I read on here about why a heater may not work, I checked the reservoir afterwards and realized that is was pretty empty so I filled it with coolant (it was diluted), since my gauge had been reading a bit higher then normal. My heater started to work, a week later was going out and my engine started reading hotter after it had been, was under 200 after the refill. We weren't to the store yet, but my engine coolant guage went closer to red and finally the light went on as it was going past the redzone (260 +) and so stopped at the closest store to get coolant (diluted) for it, since the reservoir was empty again. We started pouring coolant in and it never made the level go up and it was already half a bottle of coolant we used. Headed back home and watched the gauge, it was staying a bit down in about 240. I checked the reservoir after the engine had been off for a day and the level was not even visible. I started pouring coolant in to see if there was a leak, I didn't see anything under the car dripping. I am at a loss, would like to get an idea of the problem before we take it to the garage to get looked at.

Thanks