Every time I pass a stool I have a sharp knife – like pain on the right side of my anus?

Every time I pass a stool I have a sharp knife – like pain on the right side of my anus and then for a few hours after it is a gnawing almost unbearable pain. I get this exact condition every 1 – 2 months and symptoms last about a week, any idea of how I can fix this or what it could be ?

Thanks

Age: 20
Female
Medications: Birth Control
No