1)
What is "Nibeisa"?
1.
Radak: There is a Yud in place of the Aleph (you prophesized).
2)
Why did they object to prophesizing in the name of Hashem?
1.
Malbim: Even though the Nevu'ah is true, you should not prophesize against our will. We do not want a prophecy like this, even though it is from Hashem.
