Hallo Scorpion
If this is abnormal behavior then both of us have the same problem...
My smaller B. smithi is also a fast grower, and she is a confirmed female (Scarab sexed her for me). She molted every 3 weeks in succession and is currently sitting with a dark bald patch 24 days after her previous molt... She is at this moment about 6cm LS.
Just a bit of history on this specimen of mine:
I bought her last year in May from Petstop in Pretoria as a 1.5cm sling. At first she seemed like a typical smithi... Not that into molting / growing at all - she molted about twice during all that months! But I must say, I kept her at room temperature at that stage. I've placed her on a heating pad to try to stimulate her to eat, and she did! Her abdomen fattened up and in about a week's time, she molted. I was very pleased.
I've kept her on the heating pad and 21 days later I found her on her back! I must say... I fell for the greatest trick in the Tarantula book! I thought she was dead! :silly: Just because I did not expect another molt so quickly! I was very embarrassed when I did some reflection on that thought! Haha!
She molted again 3 weeks after that and has been keeping up since! I also thought that it might be a male, but Scarab confirmed it as a female!
I made the conclusion that heat is very important for the growth-cycle... At first I thought that the heating pad was too warm (about 45 degrees - maintaining an intra-tank temp of about 40 degrees), but then again... Mexico's temperatures is not exactly a walk in the park!
I used the same principle on other slings too, with similar results.