I don't want to belabor the Cyborg weight issue, but the excuse about her period is one we've heard before when a female missed weight. It's a convenient excuse because most of the fans in this sport are men, and when we hear it we immediately back off for obvious reasons.
Is it legitimate? Honestly, I have no idea. There are two ways to look at this. If it's illegitimate, she is doing a great disservice to other women in the sport that deal with this every fight but still find a way to make weight just fine. If it's legitimate, then perhaps we need to reconsider weight cutting for women.
There are a number of ways to solve the weight cutting problem. One of the most obvious ways would be to make fighters weigh in right before their fight. You could also institute a capping rule preventing them from weighing more than 2 pounds above their weigh-in weight. Finally, you could make the financial punishment for missing weight serious. 20% is nothing considering most of the money comes from sponsors anyway, fighters that miss weight should be docked 80% of their pay (men and women both). It's that important. If someone is making 20k from fighting, and 40k from sponsors, taking away $4,000 isn't anywhere near enough of a deterrent.
The most annoying part of this ordeal is it invited the idiots out of the woodwork to claim this proves female MMA shouldn't exist. Oddly nobody questioned whether men's MMA should cease to exist when Nick Diaz missed weight by 9 pounds. This is why it is so critical for Gina and Cyborg to put this behind them, there are already people looking for reasons to hate female fighting, and missing weight just provides them the weapon they were looking for.