I have read things like that happen often with multibit. The suggestion again was the use of some menu entry that checks the database again and repairs it. Im not sure anymore how it was named. You might ask in the thread for multibit though. Subforum alternative clients.
I've also read about it. Which wallet should I use instead?
You might use electrum though it has certain risks of connecting your bitcoin addresses. Ask me by pm if you want to know what i mean.
I would suggest using armory. It eats a lot of harddisc space because it needs bitcoin-qt too but so far it seems to be the best client privacy wise...
Oh wait... lately i found a study that mentioned darkwallet is very good too. I didnt check it out yet.