A photograph (one that would also be including the tail) would help us evaluate her age. She might be around 5 or 6 weeks but to be sure. Whichever she is, she should stay on formula until she weans herself arund 10 weeks old. what kind/brand are you giving her? If she is starting to eat solids, try to stick to green veggies for now - a baby who tasted apples and other sweets is not likely to take a liking to broccoli or kale
So start her on the green veggies, but most of all she needs to start munching on rodent blocks Henry's Blocks or Teklad 2018; that will help her teething and those are essential supplements for a young squirrel who's being raised in captivity.
Once she has developed a liking to her blocks and green veggies, THEN you can star introducing fruit. No nuts until she's at least 9 or 10 weeks, and even then just one or 2 a day.