She started drooling this morning and I dont know why?

She is a 30 pound cocker spaniel that is three years old. She has never drooled before...ever. Thats why im concerned. I wiped it from her mouth and so far it hasnt come back, but Im wondering why it started in the first place? She spotted a mouse in the house about four days ago and every night Ive had to give her benadryl so that she would sleep and not stay up pacing and looking for the mouse. Ive read that anxiety, fear and stress can make a dog drool, and I forgot to give her a benadryl last night, so could that be a reason for it happening, because last night she didnt sleep well at all! If someone could just give me a few things to look for...or some options of what it might be.