anal skin tag and pain

The knife-like description is how an anal fissure is often described. I'd get to the doctor for a proper diagnosis and (if indicated) a prescription for appropriate treatment.

As Scott says, this sounds like a fissure, and you will need to consult your doctor as it may require treatment. Of interest, skin tags can form as a result of a fissure, so perhaps this is what you are dealing with. If the skin tag itself seems irritated - perhaps from wiping or excoriation - you may want to apply a barrier cream after every BM, and/or switch to moist wipes/baby wipes instead of toilet paper until it heals up.

Of note, I have an anal skin tag, which is fairly large. My surgeon is against removing it, however, as he says removal of skin tags often creates more problems.  It really doesn't bother me, other than probably being unsightly - not that it really gets noticed down there.