Why is my dog licking her rear?

Ok, she just had her glands recently expressed and I have noticed this behavior ever since that day. Now, she did do it before she was expressed but I changed her food and then she stopped and then put her back on it. Then had her anal glands expressed.
The food she likes tends to make her go a lot. It has lots of fruits and veggies.

Could it simply be just burning her rear from pooping a lot? Or could it be it has something in it that burns her on the way out? I hate to change her food again but I thought maybe going more often was healthier for her. By the way, she was checked for worm and parasites. Nothing there. She’s healthy.
I also wonder if maybe it an allergic reaction.
Have anyone else had this problem with their dog?