What version of Filemaker are you using?
I believe that you've encountered a known bug.
For More Information see: Text Formatting Tab: Problem with upper case
This is one of many acknowledged bugs that can be found in the Known Bug List thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: http://www.4shared.com/file/8orL8apk/FMP_Bugs.html
FM 11 Advanced (MAC) I googled it first but found nothing..
Will test out changing cases etc. However, it never used to do it and has only jsut started today...strange.
changing from upper case sorted it - thanks!
I don't think I used uppercase prev. which is why I didnt notice it..