Dog started this yesterday on walks only in specific spots

Hello all this is sweet Winnie. She is my first dog and I’ve had her a month. Last night on our walk in my parents neighborhood she suddenly sniffed something on the road edge (that grey concrete part) and then kept rubbing her back into it for like 30 seconds until I moved us along. I thought it was weird but was like ok whatever. Then tonight. Same thing not in same spot - this time in my neighborhood and in the grass right about concrete - but sniffed an area and then went crazy rubbing herself around in it. Both times I looked around the spot and there’s no food or poop or anything. Just normal ground. She just went to the vet today actually for a booster shot and blood work to get spayed and they said everything looks great. I didn’t mention the incident last night bc figured it was a weird one off. She doesn’t scratch her back on things at home or any other time. So don’t feel like it’s allergies or skin rash. Idk maybe this is just normal weird dog behavior. This is my first ever pet so I’m learning