Not pooping is not a sign of constipation. When a cat or kitten stops eating, they also stop pooping. We need to find the reason they stopped eating to get them having normal bowel movements again. When a cat is constipated, they usually strain and try to defecate.
Good that her gums are so pink. Can you take her temperature with a thermometer? Normal is 101 to 102.
If I could, I would palpate her internal organs and lymph nodes, take her temp. and listen with a stethoscope. If she is not eating, she may have an infection, or fever. If she goes too long without eating, she will become dehydrated which will make her condition even worse.
Where do you live? I don't want to be scary, but years ago, I saw a kitten that was taken in as a stray. She looked completely normal, she just would not eat. She turned out to have rabies! Very rare but since you said she was a stray I thought of that.
Anytime a kitten is not eating, I would have her checked by a veterinarian.