Hello I am Dr. Joey. Thanks for trusting me to help you and your pet today. I am a licensed veterinarian with over 15 years of experience. I look forward to working with you.
I assume that she was not spayed recently; if she was spayed within the past few days then this could be associated inflammation which may resolve in a few days. Just wanted to clarify that the two are not related.
The most common cause of what you describe is that she may have a urinary tract infection (bacterial is most likely, although fungal infection is rarely possible). This is something easily diagnosed by your veterinarian but is going to require a trip to see your vet since the treatment is prescription medication.