Recently, I was gifted with a tomato seed in a plastic cup that soon sprout and grow. I placed it next to the big window where the sun often shine in from the most, and due to the sunlight was often the hottest spot in the dorm (and in the winter too with the heater.) Due to the fact that I'm staying at dorm without formal pots and additional nutritious soil, I am waiting for two weeks before I can go home and actually move my tomato seedling into a larger and more nutritious environment. For the past few days, however, it was doing great, the first pair of true leaves are actually starting to grow too, despite at a slow pace. This morning, the seedling was still doing good as well, opening its baby leaves wide and absorbing the sunlight. However, when I checked it again in the afternoon, I noticed that the leaves had suddenly drooped completely downward. That's also when I noticed that the bottom of the leaves are also purple along with the stem. I realized it might be a P deficiency. Though, the soil it was planted in should be very nutritious too and have enough of it to last until the true leaves grow larger and for me to transplant the plant into a larger pot. I checked the soil, it was a little moist but just in case, I watered a little more (not sure if that's the right thing to do.) Despite the baby leaves drooping, the tiny first true leaves still point upward and seem to be doing fine, no sign of purple coloring yet. I'm a bit freaking out, why did my tomato plant suddenly droop? My friend also had a tomato seedling that grew much slower than mine that she placed next to my pot. It was very slow in growing, and had difficulty growing its leaves out, but it didn't have any purple color nor is it drooping (though no first true leaves too.) We use the same soil and water at the same time, place at the same place. What is leading to the difference between the two of ours and is there a way to treat mine so far, or will it get back to health by itself?

I'm a beginner and I'm lowkey freaking out for my little tomato seedling buddy. Thanks a lot in advance!

PS. This is a sudden discovery. There was a dead moth behind the cup. I just cleaned it away. Could it be possible that the bacteria in the dead moth or some sort of not-good-stuffs got into my plant?