Exclamation Mark in iMessages
Does it look something like this? http://www.iqtan.com/_/rsrc/1349108748767/ios-6/9-iphone-messages/iphone-sending-and-receiving-messages/exclamation%20mark.png

This just means that the message wasn't sent, most likely because you are in an area with bad reception or Apple's iMessaging services are temporarily overwhelmed.

If not, please post a picture so we can better identify this.