Amanda Bynes -- I'm Not Crazy

Amanda Bynes is ADAMANT ... she's not crazy AND she definitely doesn't smoke weed in her car ... that's what the actress is telling close friends.

Amanda's friends have been reaching out lately because they want to help her ... but she's telling them to back off because "everything's fine."

The actress says ... the stories about her mental state "are being blown way out of proportion."

She is also telling friends ... the picture TMZ posted of her smoking from a drug pipe inside her car (below) is not what it seems. She says she was smoking tobacco ... despite the fact that her cup-holders had remnants of what appeared to be marijuana.

As we previously reported, we spoke with several people who have witnessed Amanda's behavior recently, who say she's chatting with inanimate objects ... talking to herself in the gym ... and they fear she may be suffering from serious mental issues.