How did Pocahontas die?

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 S. Ryan Johansson, 2010:

Model Life Tables tell us that when life expectancy at birth is about 30 years of age, as it was in Europe in the 1500s and 1600s, half of the ever born (not including stillbirths) will be dead by 20 years of age; but circa 30 per cent of those born will survive to reach 50 years of age, and about 10 per cent will live as long as 70 years (Coale and Demeny, 1983)

A large part of the cause of low life expectancy was death in childhood from injury or disease. Visitors from the New World to Europe would be exposed to diseases that may have been uncommon or unknown in their birthplace and to which they and their communities had not developed any immunity.

The organisation for national statistics (ONS) published some notes on life expectancy that included this graph

We can see that the ONS ascribe the improvement in life expectency to health improvements in young children ... e.g. immunisation.

This makes it relatively unsurprising that a 20-year-old visitor from the new world would encounter diseases, including some that her immune system may have been relatively unprepared for. Even if the cause of death was not such a disease but a more widespread one, the absence of modern levels of medical knowledge, treatments and care make it unremarkable that a visitor could die of disease.

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 21, 1617.

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.

See also

• The history of the town of Gravesend in the county of Kent, and of the port of London