Hi, Karen and welcome!
I'm so sorry Bella is not feeling well.
Little kittens can get into things they shouldn't and she may have ingested a non-food item or toxic substance; she also could have an intestinal infection, be constipated or allergic to something in her food. There are so many possibilities, but the fact that she is vomiting green liquid, indicates that this is bile and she has excess acid in her stomach from not eating. It can also indicate a bacterial or viral infection. When a cat doesn't eat, even for a short time, she risks developing a serious liver condition called hepatic lipidosis (fatty liver) and we would want to avoid that at all costs; however, don't offer any food at this time.
If you can crush 1/4 of a 10mg. Pepcid AC tablet very fine (use the back of a metal spoon) and mix it with the water you're dropper feeding for the moment, this might help with excess acid and nausea and soothe her stomach. Follow the dropper with the medication with one or two droppersful of plain water, to make sure it all goes down and doesn't stay in her esophagus, where it can cause ulcers. Here is more information about Pepcid AC (Famotidine):
From your description of the way she's sitting (hunched-indicating discomfort), the fact that she won't eat or drink on her own and is lethargic, plus the vomiting, it's important that you have her seen by a vet as soon as possible. If your regular vet doesn't have emergency hours, bring her to an ER vet center, now, so she can be diagnosed and treated.
The vet will probably give her an anti-emetic to stop the nausea and vomiting and if you do give her the Pepcid AC before she sees the vet, make sure to tell him/her that you did. Lab work, like blood and urine tests may be done and hopefully, she will be prescribed the most appropriate medication to help her feel back to herself.
I hope all will be well with your Bella and please let me know how she's doing and what the vet tells you. Thanks!
Please be so kind as to rate my answer with positive feedback; that is the only way I receive recognition from the site for my assistance. Thank you very much!
Providing a positive rating will not end our conversation, should you need me for any follow-up. Simply click 'Reply' for clarification or additional information, if needed, and I will be happy to continue. There is no additional charge for follow-ups.