It may be true that you can understand Flemish, and that they can understand you when you speak Afrikaans. I know a girl from Stellenbosch whose native language is Afrikaans and is studying at the University of Leuven, and she could understand and make herself understood even before she mostly switched to standard Dutch. It should be noted, however, that Flemish varies a lot from east to west, and that different Flemish dialects have more in common with neighbouring Dutch dialects than they have with each other.
Afrikaans is probably descended from Dutch of Amsterdam, but it’s just possible that there are some features preserved in both Afrikaans and Flemish that make it easier for you to understand. I’m a bit sceptical.