I don't think much more is known. Medical diagnoses were probably much vaguer in those times and detailed records were not so commonplace and would not all have survived.
It seems strange that she should die so suddenly at so young an age
Why? In 1617 death from illness was more common at all ages. It was not that uncommon for travelers to other continents to fall victim to diseases not present in their country of origin and for which they had no immunity.
According to Wiklipedia: "During the early 1600s in England, life expectancy was only about 35 years, largely because two-thirds of all children died before the age of four". So an English native who survived early childhood diseases might expect a longer life, but the same could not necessarily always be expected for immigrants or visitors who were born in America.
According to The Story of Pocahontas by Charles Dudley Warner
Camden in his "History of Gravesend" says that everybody paid this
young lady all imaginable respect, and it was believed she would have
sufficiently acknowledged those favors, had she lived to return to her
own country, by bringing the Indians to a kinder disposition toward the
English; and that she died, "giving testimony all the time she lay sick,
of her being a very good Christian."
The Lady Rebecka, as she was called in London, died on shipboard at
Gravesend after a brief illness, said to be of only three days, probably
on the 21st of March, 1617. I have seen somewhere a statement, which
I cannot confirm, that her disease was smallpox. St. George's Church,
where she was buried, was destroyed by fire in 1727. The register of
that church has this record:
"1616, May 21 Rebecca Wrothe
Wyff of Thomas Wroth gent
A Virginia lady borne, here was buried
in ye chaunncle."
Yet there is no doubt, according to a record in the Calendar of State
Papers, dated "1617, 29 March, London," that her death occurred March
According to The Smithsonian
The copper-plate engraving, by the Dutch artist Simon van de Passe, was published in a volume devoted to English royalty. The inscription beneath her image makes clear the portrait’s message: Matoaka, daughter of an Indian “Emperour,” had been “converted and baptized,” becoming Rebecca Rolfe, a respectable, thriving and thoroughly Anglicized lady.
But look closely at the portrait. Pocahontas appears grave, her cheeks are sunken and her hand is skeletal. Perhaps this was simply the artist’s rendering. But it may have reflected her failing health. In common with so many natives exposed to Europeans in this period, she and her young son fell ill in England, possibly from tuberculosis. Soon after the Rolfes set sail for Virginia, Pocahontas had to be brought ashore at the Thames port of Gravesend. She died there in March 1617, at the age of about 21.
The National Parks Service say
In March 1617, the Rolfe family was ready to return to Virginia. After traveling down the Thames River, Pocahontas, seriously ill, had to be taken ashore. In the town of Gravesend, Pocahontas died of an unspecified illness. Many historians believe she suffered from an upper respiratory ailment, such as pneumonia, while others think she could have died from some form of dysentery. Pocahontas, about twenty-one, was buried at St. George's Church on March 21, 1617.