WNBA star Kelsey Plum is having a pretty crappy Monday ... and she says it's all 'cause she drank a tainted Jamba Juice smoothie!
The Las Vegas Aces point guard took to Instagram to explain her plight ... writing on her IG Stories that she believes she's battling some bad diarrhea this week due to some extra berries she threw into her JJ drink over the weekend.
"Peeps," she wrote in a warning to her followers, "stay away .