Amanda Bynes was arrested in Los Angeles Sunday after cops determined she was driving under the influence ... and our law enforcement sources say she was on a STIMULANT ... but we're told she has also developed a serious weed problem ... again.
Here's what we know -- Bynes was driving a Mercedes in the San Fernando Valley when she stopped in the middle of an intersection on Van Nuys Blvd.
A CHP officer spotted her and determined she was under the influence. She was taken to the LAPD Van Nuys division where a drug recognition officer determined she was under the influence of a stimulant. Toxicology tests are being performed now to determine the exact drug or drugs.
According to the incident report ... Amanda had a "disheveled" appearance.
Amanda was then booked for DUI. She posted $15,000 bail.
When you think this girl was done with trouble it starts right over again !!
