message with text i didn't send

Hi everyone,

something super weird has happened. I was text messaging my partner and he received the exact text message I wrote up to the last three words...and then a completely different end to the message (it added a whole sentence I didn't write instead of my original last three words). This has happened once before months ago and I was a bit spooked then, changed all my passwords and forgot about it...until just now. Does anyone have any idea what could be going on?