
I'd get her checked out, if this is something she doesn't normally do.
It took a similar thing for me to get Soli checked out - she guards beds and sofas, but she snapped at me once when she was lying on the floor, somewhere she doesn't guard. I went to stroke her side - slowly, and she watched the whole movement - and she snapped. I got her checked, her right hip was sore; the chiropractor found a wonky pelvis and vertebrae out of whack and she's been much better ever since, including with the bed guarding and other dogs.
Also keep an eye on her signals - Sol is a difficult dog to read sometimes, so the snaps sh ehas given have been my fault for not responding porperly (i.e. backing off). Look for stillness, sideways looks at you, licking lips, lip lifts (may be very small), anything like that. What makes Soli difficult is that she'll go still if she wants me to back off a bit, but also if she wants a fuss - with her I have to look for a very, very small lift of her head towards me to let me know it's okay.