Your doctor must have been a reject from some Central American medical school. It's fairly common knowledge that women are incredibly fertile following a childbirth.
It is?
Nothing I've ever read supports this. It is a myth that you are
not fertile after childbirth-- there are plenty of siblings born 9-10 months apart. Studies show that breastfeeding does reduce fertility BUT not reliably and women who rely on that will most likely end up pregnant. This suppression is dependent on the strength of suckling by the child and can be very unpredictable.
However, the statement that you are "incredibly fertile," meaning more fertile than usual just isn't true. It does take some time to get back to normal and have proper hormone levels to ovulate for most women so if anything, fertility is reduced. However, no women know if they happen to be one of those who ovulates two weeks after childbirth or 6 weeks, or longer. Even when you have your first period after childbirth, it's not guaranteed that you ovualted beforehand.
There may be a misconception that childbirth makes you more fertile simply because ovulation and fertlity are somewhat random as a woman's body goes back to normal.
Finally, don't denigrate "South American medical schools". There are plenty of very skilled physicians that come out of every part of the world. If you are referring to the schools in the Carribean that generally cater to students that can't get accepted into US medical schools, many of them are accredited in the US, so the training is at least somewhat upto par.