After her husband dies, the narrator speaks of two terrible things, which he should not mention, that happened to Hunilla. What are those things? They are buried in the semantics of Melville's writing, but: 1.) Hunilla actually becomes pregnant and has to try and have the baby herself because her husband and brother died on the catamaran. 2.) After she gives birth, the baby dies, and she is raped by men on a ship that boards the island. They leave after they rape her, leaving her alone again.
Not too long afterwards is when the other ship saves her. These events are implied and hidden in Melville's writing so they are very hard to sieve out. One way you can tell she is pregnant is by adding up the months she marked with her reed (9 months) after her "natural calendar" (period) stopped. She implies that a ship besides the one that saved her landed on the island when talking to the captain. It's kind of a stretch to say that those who landed on the island before the captain did also raped her, but... yeah.