a vertical rectangle shape is merely a symbol or a emocon, or any character that is in a code that your phone cannot decode. usually a chinese, or arabic, or korean characters will undergo this "failure to decode".
if the whole message is in rectangles, than that means the entire message was in a different language.
if only one...than it could be some very intricate happy face.
depending on how advanced your phone is, you can try copying the text to a SD card, if you phone has the capacity or any other means to get it to a computer to see if it can accurately decode the text.
if you have a friend with a smartphone, perhaps you can foward the text message to her and see if her phone has the ability to materialize it accurately...
i'm not sure why you can interpret that something is going on behind your back from one rectangle...
hope i was able to help