How likely is my girlfriend pregnant when she loses her period on the first month use of the contraceptive shot?

I head that only 39 percent of women loses their period by the 6th month or the 2nd dose of the contraceptive shot. However, my girlfriend lost her period starting immediately after the first month after using the shot. Before the shot, she used Diane 35 combined contraceptive pills.

Prior to having sex (and me ejaculating inside her) we waited for 7 days after she was administered the shot to be sure that she is protected. I also noticed that she has gained a lot of weight.

1 month after the shot her stomach grew, then 1 month after that her stomach grew again. But I noticed that she gained a lot of weight on her butt and thigh area as well, so if gaining weight on the butt and thighs is not a pregnancy thing, then I would say she just gained a lot of weight in 3 months while using the shot.