Hello,
I've used Monistat 1, and have never experienced what you've described. Monistat can cause side effects (most commonly burning sensations,extreme itchiness and excessive discharge), but gritty sounds a bit unusual. Get in contact with your doctor again, and explain what is going on. She might suggest that you come in for more tests, or another exam. You may want to abstain from sex until after this clears up.
Take Care