Kat's reps say she merely had food poisoning from eating at a restaurant outside of the resort.

Kat Graham got way too high, according to witnesses, and ended up in the hospital ... TMZ has learned.

"The Vampire Diaries" actress and singer was visiting Two Bunch Palms Resort & Spa near Palm Springs last month when she allegedly started throwing up and passed out on a lawn. It wasn't food or booze that put her there though -- sources at the resort tell us she'd downed a cannabis brownie earlier in the day.

We're told Kat ran out of the resort's restaurant saying she didn't feel well and needed to go to the hospital. She puked, passed out, woke up and then hopped an ambulance to a hospital.

The resort's report said she was extremely paranoid, anxious and seeing things. Some people call that a really good trip ... not so much for Kat.

We reached out to her people, but no word back.