You’re doing well with the kitten so far. However, there are some things I’d consider doing, if I were you.
1. Vet visit – This will be relatively inexpensive, as the vet will just be checking over the kitten, possibly looking at a thing or two under a microscope, and talking with you about what the kitten’s doing, and what you’re doing. This sort of professional evaluation and advice is worth every single penny!
In addition, the vet can advise you on when to bring the kitten back in for vaccinations, as well as what kinds of problems might indicate a need for a return visit. (When unweaned kittens become sick, they can often become very ill, very quickly.) There’s also a bacterial supplement you may be able to feed the kitten, which will help substitute for the natural immunities that the mama cat would normally be passing on via breast milk and grooming.
2. Feeding – Are you feeding the kitten a commercially prepared kitten formula? If so, poop should be very soft and gushy, but probably not yellow. And the kitten shouldn’t be crying to pass it! When you pee & poop the kitten, you should be taking a slightly damp, warm washcloth and gently massaging the kitten’s genitals directly. (Gentle is the key; you don’t want the kitten rubbed raw.) You’ll want to do this both before and after eating. The kitten should both pee & poop into the washcloth, which is what you want. If the kitten is straining or crying, there may well be a problem – another reason to see the veterinarian.
(Note that rubbing the kitten’s tummy isn’t really what’s needed, and that the genital stimulation is as necessary as proper food, warmth, and air to breath; there’s nothing optional about it.)
3. Tiny kittens don’t usually overeat. When the mama kitty is present, they nurse nearly continuously; they’re attached to a teat at least 50% of the time, and eat a tiny amount all day long. When in doubt, feed more often, not less often.
Please, seriously consider taking the kitten to the vet. You may be surprised at how much useful information and feedback you will get!
Good luck!