Maybe it's just my eyes, but in all three pics it looks as if the stem that meets the soil just as its going into the soil is extremely pinched looking. I'm betting if you were to pluck that seedling out of the soil right now the root system would be almost nonexistent and the stem would probably break off at the soil line. Which does NOT bode well if that's the case.

Not that you should pluck it out of the soil, give it some more time, but I'm feeling a bit scared for that seedling. But the top of her looks good though. I'm thinking (guessing) pyrethium wilt at this point. She doesn't look overwatered, so maybe it was soil borne? It's hard to say. She doesn't look like she's stretching, so much as her stem is just wasting away.

I do hope that she pulls through