Spellcheck on touchscreen interfaces is notoriously annoying, even on iOS. You think you typed one thing, send your email or text message, then realize the spellcheck chaged a word or two, often very awkwardly! Haven't checked in the Fire yet, but most OS have settings where you can make it behave better or simply turn it off.

EDIT: You can turn this On or Off by selecting Quick Menu --> More --> Kindle Keyboard, then for the Quick Fixes option select On or Off.