Evolution gives "erroneous" turns only insofar as the changes make the survival of the species impossible. That a feature is not the peak of perfection (however we want to define that) does not make it an evolutionary aberration ; if it has its disadvantages, but allows the continuity of the species, then, look, what stays!
For it is that evolutionary developments are not, in general, improvements or arrangements; they are adjustments . Nature does the best it can with what it has.
Maybe those fingers will help you run the little lizard, but what if we join you for a few telitas? You no longer run so well, it's true, but now you're fleeing! If you come to eat the tree you can now jump! *
We already arrived at your first question.
If menstruation is a natural process, why do women experience pain and discomfort?
First, the obvious. Nature never promised that its processes would be free of pain; just ask any organism that came to rest in the stomach of another.
And second: because menstruation is just another adjustment. Menstruation (among other processes, of course) is the response of nature to the question "how carijos prepare this body to lift a puppy?" It is not perfect, I give it to you, but it works and evolution only needs to work.
The day that women begin to fall dead up and down for menstrual pain, there you can begin to doubt the wisdom of nature.
(*) Please, do not take this as a scientific example. I invented it without a bit of research or reflection.