What do you mean by "a spy thing"?
It's right, in a way...com.android.mms is a package that includes the software used by messaging apps, and it can perform a check to see if a message is SMS or MMS. It's on every Android device, this isn't some package that someone installed. I don't know of any way to tell exactly what happened in your case. Some messaging apps (Hangouts, for one) can force SMS into a MMS when there are multiple people in a group. I don't recall the specifics of why it does this, though.
Essentially, all that saying "your message has just been read by com.android.mms" means is that your phone opened a message. It doesn't mean that someone else had access to it.