Sounds like the beginning of corruption...
http://www.granite.ab.ca/access/corruption/symptoms.htm
I would make a back-up, make another back-up and then first try importing everything into a new database. Also, read this article...
http://www.granite.ab.ca/access/corruption/retrievedata.htm
And after all of that I would finish off with this article…
http://allenbrowne.com/ser-25.html and see if any of those circumstances apply to you.
And when all else fails…
Data Recovery Services (I have never used them but I see them most mentioned when trying to recover data)…
http://www.pksolutions.com/t4.htm
Note, after you recover (and hopefully you do) please make backups before and after making changes. You should also be making regular backups if the database is in use that way, at most, you only lose a days worth of data.