What made my hard ice tea go south?

I brewed a 4-gallon batch of what was supposed to be hard iced tea. The day I bottled it I sweetened it with cane sugar and it tasted exactly like Lipton Brisk Iced Tea from the can. It was exactly what I was going for. By the next day the flavor changed quite a bit. The iced tea taste went away for the most part leaving an odd lemonade taste. It doesn't taste bad, but it doesn't taste like anything I can identify. The recipe was simple:

Steeped 4 1-gal tea bags in one gallon water
Boiled 2 quarts water with 2 cups lemon juice (from lemons) & 4 pounds cane sugar & 4 tsp nutrient.
Added to 4-gallon carboy with a gallon of water in it.
Topped off for 4 gallons with water.
Pitched Red Star Champagne yeast starter at about 70 deg F.
Fermented a bit warm at around 75 deg F. for 15 days.
Bottled it in PET soda bottles.

What would make the taste change so quickly?

 

I failed to give you all the info...

It was dry as a bone...0.992 two days in a row. I planned for it to be drunk right away, so I placed the soda bottles in the refrigerator. Only after they were cooled to refrigerator temps did I add sugar and only to one at a time. The original taste (the sample that tasted like Lipton) was taken straight out of the carboy, sweetened, iced and imbibed.

I siphoned it out of the carboy with the siphon tube all the way to the bottom of the bottles. I filled them up to 1" to 1/2" from the cap. Even though I did that, my guess is that oxygen got to it. I shook the bottles to mix the sugar.

I might try it again and this time try and fill the bottles with CO2 before I fill them. It was quick, cheap and the first taste I had was great. I'd love to be able to recreate that.

 

I would do it again the same as before, bottle as is and cold store them, then sweeten your glass as you would regular iced tea, no need to pre-sweeten! or maybe sweeten with splenda at bottling, since its not fermentable it shouldnt change flavor wise after bottling. thats just my thoughts!