iPhone 4 is sending texts that I am not writing

I don't know how long it's been going on, but apparently only over the last day or so my phone has been sending out texts that I am not writing or sending. So far, only one person has told me they are getting them, so I don't know if my entire contact list is geting them or not. I know for sure a few contacts are not.

The phone receiving them is not an iPhone, and he is telling me that the texts are either completely blank, or are telling him to go to a website to look at the message. When he does this, there is nothing there. The website coming up for him is viewmymessage.com.

I am for sure not texting these; some go out in the middle of the night when I am definitely not anywhere near my phone. Any help would be really appreciated; I'm getting worried about this and any information getting out there.

Thanks in advance!