The previous month is last month.
This is a jukugo word, which usually means on'yomi readings from the kanji. Since 月 has two on'yomi readings, here's a mnemonic to help you remember which one to use:
As of last month, your centaur gets (せんげつ) up every night and comes to sleep in your bed. Apparently, he gets really bad nightmares. Poor little centaur.
Last month was June.
Last month's portion was 10,000 yen.
We met in New York last month, right?