Assuming there is no written lease and you have been paying once/month at 150, you should have 30 days from the next pay day after getting notice of the rent change. You need only pay the 150 for that 30 day period. After that, you need to pay the new rent or leave.
Mr. Rensch is correct. The reason for his answer is that a verbal lease is allowed when it is considered a month to month lease. What that means is that you only really have a lease for a month at a time. Each month the rent could be raised. On the other hand, each month you could choose to leave without owing for additional months. That is why your roommate can up the rent on you.
The flip side of this is that you are also being offered a contract with the new increased rent. That could be a double-edged sword. It would mean no additional increases during the term of the written lease (unless the written lease is also month to month); however, it also commits you to a longer period of time when you are required to pay rent.
Sadly, this is an expensive but good lesson. First, you have to get a better feel for your roommates. Second, you have to approach roommate situations more formally even if they are friends. Third, always get it in writing.
Disclaimer: This answer is provided for informational purposes only, does not constitute legal advice, and does not create an attorney-client relationship. I am licensed in Arizona and can only provide general comments on matters outside of Arizona law. Actual legal advice can only be provided after a direct consultation in which all of the relevant facts are considered before providing a response.