my iPhone 5 says that there is no sim card or that it is invalid

Several times since downloading ios 7 my phone has given me the message that there is no sim card or that the sim is invalid. I have to turn it off and back on to get it to register a sim. This usually happens after I take it out of my pocket, though that is where I've always kept my iphone. I'll look to see if others are experiencing this, but couldn't see anything before the screen brought me to this.

Thanks for any help you can give,

Michael Richardson