Seedling Cotyledons with brown spots

Sorry guys, I've browsed and searched the forum and couldn't find anything specific to this problem. There is a picture attached. This is my first time trying to grow, just a bagseed that I sprouted. It's outdoors all day, at the 33rd N latitude in Texas. I know it is late for outdoors, but I don't want it getting too big. It is planted in Kellogg's Organic Garden Soil, in a Solo cup for now, receiving about 6-8 hours of direct sunlight a day and the rest it is shaded by neighboring plants. I am watering with bottled water from the local supermarket. "Purified drinking water" is what it says. Anyhow, can anyone help me maybe identify this problem, given the info?