I'm looking at the angle of the book lungs as you've noticed, and it looks to have a bump at the uterus externus. Also, it looks to have the tale tell hardened lower lip at the epigastric furrow.
See pages 195 and 196 in the Tarantula Keepers Guide..
It's probably female, but since it's a small specimen, I'd like to see the rest of it. Brachypelma are nice in that females develop their tell tale larger chelicerae early on as well, makes sexing the smaller ones a bit easier because you have more than one external sign to go on.
Males, ime, with young Brachypelma tend to have very straight book lungs, so that's the main indicator as, in spite of what was said above, I don't see any sign of any real sclerotization of the epigastric furrow yet on your specimen.