Mary's cruel father Henry VIII caused to her those healthy problems, she wasn't married until 38 but she couldn't have children.

In the book, she is going through a particularly painful period. She was never healthy and sufferen from gynecologicak problems all her life.
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Everyone is replaceable. Even you.

I looked it up online and all I could find is that she suffered from "irregular menstruation", so possibly Endometriosis or hormonal disorder.

Yes - there is no "correct" spelling. Like her contemporaries, she alternated the spelling when signing her name. Her name at birth was Catalina, but her most famous signature is "Katharine the quene". Spellings such as "Kateryn" were also used. Most writers just use the standard spelling "Catherine" for consistency. After all, it doesn't really matter all that much.

Actually, there was no standard spelling at the time. If you look up paintings of Catherine of Aragon, her name is spelled with both a "C" and "K".

You know, I've often wondered about that. Properly, her name certainly should be spelled with a C, as she was technically Catalina of Spain, and Catherine was the more common spelling of the English version of the name even in that era of woefully inconsistent spelling.
And yet, we have all these records of her and Henry's intwined initials being "H&K," so I don't know what that's about. Perhaps the artists doing the initials just thought it looked better that way? And on her tomb it reads "Katharine." Anyway, that's how it's stuck in my mind now.
Formerly Nothin_but_the_Rain

To moss_garden:
Small detail *** Just a small spelling correction it's Catherine, with a 'C'. I thought the same as you but happened to look it up online to confirm the spelling for a post I was writing . . .

I didn't see Katharine being anything like a cold-hearted shrew in this depiction. She's a dignified, wronged, intelligent, loving wife and mother. She's not real warm to Cromwell, considering he's helping Henry to dump her to marry Anne, but that's understandable. And he tries to get permission from the king for her to see her daughter, or Chapuy before she dies, but no luck.

It was complete tosh.
Katharine of Aragon was a compassionate woman who smiled even in the midst of her troubles and adored her daughter; turning her into a cold-hearted shrew just to make Cromwell look good is an injustice.

Ah, ok, got it, thanks.
AVADA KEDAVRA!!!

That wasn't Mary, Queen of Scots. That was Mary Tudor, Henry and Katherine's daughter, later Mary 1 or Bloody Mary. Historically she was a sickly person, never in good health for one reason or another. The show used the quickest way possible to show that without having to make a big deal out of it. Also it showed how strict Katherine was even with her beloved daughter, and gave Cromwell an opportunity to show Mary a little kindness.
Formerly Nothin_but_the_Rain