Please Help. Dog Snapping at me.

I have a 5 year old dog who is generally great. Love her to pieces. She is on the shyer side but never had any significant issues. She resourced guarded a little as a puppy, and has snapped at me / people in the past if she was sleeping and you went to move her or anything but nothing “unprovoked”, or if you gave her space.

Last night I went to put her harness on and she let out a loud yelp. I was scared I pinched her or something (but I’m pretty sure I didn’t, I don’t even think i came close to buckling it). Anyways I tried to put it back on after and she lets me put it on her, but if I tried to buckle it and reach under her she shows her teeth and me and snaps at me. She is incredibly food motivated, but even bribing her with treats didn’t work. I figured I’d wait until the morning and try again.

Same deal in the morning. Maybe she sensed I was nervous? Either way, she let me put on her collar. I actually took it on and off a few times to size it, and she was totally fine with that and I walked her via her collar.

What do I do? I am freaking out because our family is supposed to be watching her a decent amount over the next couple months and I am incredibly nervous something will go wrong.