Mine does the occasionally as well.. :/
It only does it for little while then it goes back to normal, it isn't because of rsbuddy or nexus because it does it on all clients I use. It seems to be a small inconvenience.
If you don't want to lose your rapier then I would definitely recommend buying just a cheap mouse for the time being, if this doesn't work then check your ingame settings
This same thing happened to me when I was using Rsbuddy, and I almost lost c maul. My p pot dragged, so the only things I could do were eat or tele. Luckily I just hit a tele tab.