I Consider My Pets Family

Question for those with dogs
My dog ordinarily, loves going for walks !
Runs to the door when you mention the word. Pulls at the lead as you walk her. Loves to stop and sniff certain places.
Tail wags incessantly even after she's been back ten minutes !
But, the last few days she has refused to go beyond the front door !
She'll stand there and sniff the air, then run back into the living room refusing to come out !
Real change in character. Almost as if something had spooked her !
Any ideas ??