Why does my iPhone say invalid sim?

Ever since I got one of e software 5 updates my phone will randomly say invalid sim or "no sim card installed". When this happens I can't text, call or go on the Internet because I can't get wifi. I went to the store and they said it was because my sim card doesn't work but I don't think that's the problem because it will work for weeks at a time then for 2 or 3 days it will constantly say invalid sim. I googled this problem and some people said it was the software update but the guy at the store doesn't believe me. I'm getting really frustrated because all day I haven't been able to use my phone.