Lindsay Lohan at Coachella -- DENIES Boozing and Drugging

Lindsay Lohan insists she was clean as a whistle at Coachella ... navigating around the muddy waters of drugs and booze.

LiLo is telling friends ... she didn't drink an ounce of alcohol and stayed clear of all drugs ... even though people all around her were getting high.

So why should we believe her, because frequently we don't? Here's the thing. There are NO pics that have surfaced of Lindsay partying, and whenever she does ... something gets out.

In fact ... we know Lindsay was invited to numerous parties and we checked with people who attended -- and could remember -- and no one saw her.

Lindsay is telling her friends she turned in by 10 PM both nights during the weekend.

Raising the question ... why bother going?