fertility postpartum

I'm almost 4 weeks postpartum and am excited for activities in the bedroom to resume as normal since sex was so miserable during my third trimester... and the end of my second. I've been told that it is especially easy to get pregnant again right away after having a baby. I know it does happen sometimes, but was curious as to why? Is there a real medical reason for why a woman is so fertile in the weeks/months following childbirth, or is it a fluke thing? Calling all ladies with any experience or medical knowledge