I'd definitely run a recover on the file to check for damage. The "lock up" may be due to a damaged file or the force quit could have damaged your file and file damage isn't always something that can be easily spotted when using the file.
What you report seems vaguely similar to this report: FileMaker enters a modal-like state on editing
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: https://www.dropbox.com/s/jt09b82i0xijbu3/FMP%20Bugs.zip
Do you see any common factors? Any differences?