Heat??

I’ve never experienced a dog in heat before. Poppy is 3 years old and we adopted her almost a month ago. We had no vaccination history, so we started her off basically like a puppy with a vaccine schedule and she’ll be fully vaccinated on the 3rd. Our vet said vaccines first, then spay. I noticed her backside a little bit bigger yesterday and her licking her hoo haw this morning. She’s a bit lethargic and grumbly too. Could she be starting a heat cycle? What should I expect? Can I still walk her, or should I just keep her “safe” in our house and yard only?