RIHANNA says she's suffering from LARYNGITIS...and because of that, she was forced to cancel her show in Boston last night. The show will be rescheduled.

Rihanna Tweeted, quote, "Boston: This is the hardest thing for me to deal with! I feel like we've been waiting on this day forever, and I'm hurt that I let you down.

"I hate disappointing people that never ever let me down!! I'm so embarrassed about this! Thank you for your prayers and well wishes!"

For now, she's still scheduled to perform in Baltimore tomorrow night.