[HELP] My dog has gotten SUPER possessive/territorial lately!

Hi, there! Long-time lurker, first-time poster asking for help.

My dog(GSD mix) just turned 5 this passed March and has all of a sudden gotten really aggressively possessive of me. My SO and I sometimes let him cuddle on the bed with us but lately he's gotten really snappy and growl-y at my SO when he tries to sit down or shoo him off.

I should also mention that I crate him up in the office when the both of us are working. Tonight, while I was working on my website(and redditing, duh), my dog was just sleeping in the corner. My SO comes in and the dog just LOSES HIS SHIT and gives his warning barks to him! We crated him immediately, of course, but this shit has got to stop.

The dog's behavior is VERY new. He nuzzles his way in between us when we kiss now and it's getting really baffling.

I've had my dog since puppyhood and my SO came into the picture about two years ago, so it's not like the SO is a new person. We've all lived under the same roof for about 9 months at this point so I really am baffled.

ANY help AT ALL is greatly appreciated. I want to nip this problem in the bud before it becomes dangerous.