I agree with your vet. Sudden changes in behaviour or obvious symptoms like your vet said will tell you.
Don't worry too much about it. Just make sure you tend to her needs (including love and companionship) and worm her, make sure she's not got fleas or ticks and take her to the vet once a year for a check up and I'm sure she'll be fine.
As you grow close to your dog, you'll know what's normal and what's not.
By the way...the fact that you're so concerned tells me good things about how much you love and want to care for your dog.
Cass.