I'm guessing (we had lots of barn kittens when I was growing up) that it's got dystemper. Our kittens always came through it as far as I can remember.
Stray kittens will cry a lot, especially when you leave them alone. If it crys even when you're petting it, and it has moist food/water (NOT MILK), they you might want to take it to the vet.
I'd take it to the vet regardless, especially so you can get it shots. Feline lukemia is nasty so you'll want to get those started ASAP. We had a stray cat bring feline leukemia around our cats and we had litter after litter of kittens die from it every year, we finally started getting them vaccinated, and it finally worked its way out of the soil.
And check the kitten for fleas. Check it VERY well. We've had many a dropped off kitten come to us with fleas, which can cause all sorts of problems.