I always noticed the same thing. I suspect it is a combination of improved confidence (which women usually find attractive) as well as a hormonal response.
A study was done on photographs of various men with their testosterone lvls being tested, and there was a definate corrolatoin between the test lvl of the man who's picture was taken and the number on a 1-10 scale women viewing the pictures gave the men on their sexual attraction to the men.
I believe women can also sense the higher androgen lvls as well, and it has some effect on their libdio when arround you.
To make a long story short, when I was on a hefty amount of test, I always have noticed women tend to throw themselves at you and flirt with you a lot more in public.