I'm gonna give an example of my idea.

Guy 1 PM's you.
You typing to reply to him
Suddenly, someone else PM's
You're reply goes to the wrong person


Try to change it to /r [ID] [Text]
To evade problems for example someone doing this to another guy who didn't pmed him, something should come up saying ' ID [Number if it] didn't pmed you '

His got a point there, if it just shows the most recent PM's that you haven't seen - that would be much better.