"Ugh, stupid uterus" has to go on our list of Stupidest XKCD Quotes, along with "do me without a condom." Randall just doesn't know how humans talk, or at least human females. He has to quickly get across the point that this woman is on her period, and he can't think of any better ways, and so he goes with the "just come out and say it, who cares about whether it makes sense" method.
He also makes sure that we understand he has made a funny in panel 4, via a similar method of "Have the other character get angry, so you know a bad pun just went down." Randall just because one character says it's a bad pun does not absolve you of the fact that your comic just got its laugh (if the reader laughed) from a bad pun. You can't just replace good humor with bad humor and apologize, expecting that to be enough. It's not.
Also, and again this is one of those things where I'm not a woman, but are women on their period actually incapable of motion as implied by "If I could get up..." ? I have known a small number of women and I imagine they had periods at some point but I don't think any were actually rendered immobile. I don't think.
Guys what else should go on the "dumbest sentences ever uttered in xkcd" list? we can make this a thing.
update: Ah crap, I forgot I meant to say something nice about all these. Um...if I get in the right frame of mind where I like terrible puns, I can vaguely enjoy the feeling where you read the phrase "uterus-hertz" and do that sort of mental double-take and realize its second meaning. Of course that's true for all puns, but still, I promised I'd try.