Why Apostle Peter calls Paul "the man who is my enemy"

I read from "The Letter of Peter to James" in After the New Testament: 100-300 C.E.: A Reader In Early Christianity by Dr Bart D. Ehrman https://www.amazon.co.uk/gp/product/0195398920/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1