Stealing my own powerarmor

There are only a handful of unclaimed power armors in the game, and those are usually locked up or hidden. You can also buy an unlimited frames from vendors.

You can't loot it from dead bodies.

If you shot the fusion reactor (bandit or BoS), it is still "owned" by whoever that was using it, and stays that way permanently. Even if they were forced to leave their power armor via the console, the armor is still owned by them.

It most likely was stolen to begin with and you didn't notice. It is best to use another power armor frame and let Nick keep that stolen one. You can, however, "steal" the parts off the frame and if they were not stolen, it doesn't register as stolen parts.