(Public domain image sourced from wikipedia).
Although she recognised herself as the his first wife, Henry himself did not do so, styling her only as the Dowager Duchess of York, a title she held when married to his older brother Arthur. Their marriage had been declared invalid based on the idea that a man should not marry his brother's wife, some three years prior. This had paved the way for Henry to marry his long term mistress, Anne Boelyn. It had also disinherited Catherine's daughter, Mary, a fact which caused her great distress and one she refused to recognise.
In her letter Catherine implored Henry to protect their daughter and to arrange marriage portions for all of her servants. She forgave him stated that she was praying God would also do so.
A month later on the 7th January 1536, Catherine died. She was fifty years old.
At the time of her death her relationship with her former husband and his new wife was somewhat strained. Catherine proudly never stopped seeking what she felt to be her due and continued to style herself Queen, refusing to recognise the dissolution of her marriage and the declaration of her daughter as a bastard. She remained immensely popular with the public, a fact that irritated both Henry and Anne. Anne is recorded as stating multiple times that she wanted to murder both Catherine and Mary. Henry for his part ensured that mother and daughter remained seperate, even as Catherine lay on her deathbed. He had Catherine exiled to isolated Kimbolton Castle and controlled her visitors and staff.
This strained relationship with the King and his new wife was undoubtedly the source of the poisoning rumour that spread rapidly on Catherine's death.
Catherine had been ill for some months. Visitors described constant vomiting, wasting of her muscles, weakening and stomach pains. Then came the report of the embalmer that removed her organs after her death. He noted that they were all healthy - apart from the heart. The heart was blackened with a large black growth off one side.
Based on understanding at the time this was a sign of foul play.
Reactions by Henry and Anne divide historians. Anne chose to wear yellow on the day the news of Catherine's death was recieved. This has been variously interpreted as being a slight, or embracing the culture of Catherine's home country Spain, where reportedly yellow was the colour of mourning. Henry apparently celebrated the news by showing off daughter Elizabeth to his courtiers. Mary was strictly forbidden from attending her mother's funeral.
However both Henry VIII and his wife are reported to have privately wept for Catherine and not long later Anne miscarried a baby boy. Again this could be variously interpreted to be as a result of extreme grief or stress over the public backlash. Or it could have been for entirely unrelated reasons.
At any rate, although the death of Catherine removed problems for Henry VIII and Anne Boelyn it in essence created a PR nightmare and the theory of poisoning persists in various works of fiction until this day.
However is there a better theory?
A extraneous growth, anywhere in the body, under modern understanding implies a tumour, not poisoning. The embalmer can be forgiven for not knowing this as cancer was not a diagnosis that was recognised during the Tudor era. The fact that it was noted to be black implies that it contained a pigment named melanin.
Any tumour that arises from connective tissue, such as heart muscle is known as a sarcoma. On the whole sarcoma's, particularly on the heart are quite rare. What is a common cancer, now and then, is melanoma, an aggressive form of skin cancer. Melanoma is known to frequently show up as a secondary cancer, or metastasis, in the heart. As the tumour grew larger it would have impaired Catherine's cardiac function, causing slow decline and death.
Not that poison still isn't possible, of course. However Catherine and her heart are long gone so all we are left with is conjecture.
Whatever the matter, Henry VIII and Anne never managed to shirk the stain of responsibility for her death. Anne particularly felt the disdain of the public towards her only deepen. Indeed Henry's guilt over Catherine's death and the possibility of poisoning may have added to Anne Boelyn's downfall.
Not that it mattered as in a sense Catherine had her revenge. Anne Boelyn was beheaded only a few months after her death. And her daughter did go on to rule England in the appropriate line of succession, albeit only for five years and with a doubtful legacy.