EXCLUSIVE

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.

Amanda has had trouble with mental problems for years now. Her parents had a conservatorship which ended earlier this month and as one source says, "Things started going haywire."

We're told Amanda has moved out of her parents home and is now living in an Orange County apartment. We're told she has been smoking weed for weeks and things are bad again.