In case anyone is curious about why, her party (the Social Democrats) was the senior party in a coalition with the Green Party, Centre Party and Liberals. They had drafted up a budget, but the Centre Party and Liberals thought it was too left-wing, so it failed to pass. An alternative proposal drawn up by a coalition including the far-right Sweden Democrats passed instead, resulting in the Green Party withdrawing from the coalition in protest, and it's apparently tradition in Sweden for a prime minister to resign if they're part of a coalition that loses its majority.

Anyway, as the Social Democrats are still the largest party in Swedish parliament, she could well be the prime minister again in a couple days.