Thank you for your query
The bleeding can be due to a variety of causes. She may be having a fissure (cut) near the anal opening. I hope she is not constipated. She may also be having the onset of periods. Illnesses like these may hasten it. It is unlikely due to amoxycillin
as it is not a known side effect of this drug. However severe infections themselves may cause fall in platelet counts and and cause bleeding. I suggest you consult your treating physician regarding the same. Hope fully it will turn out to be something minor. You can give her Sitz bath i.e make her sit in a tub of luke warm water for 10-15 min, 3-4 times everyday. It will help if she has fissure and also help maintain her hygiene.
Hope that helps
God bless you