random capitalization of words

Recently inDesign has begun to capitalize the first letter of the words I am typing while editing text that I have written previously. This has never happenned to me before but now it happens routinely. Usually if I go back afterwards and change the capital letters back to lower case, they will stay that way. But sometimes they revert back again. I have no idea why this is happenning. Please advise if you have any ideas. Thanks