Since you just had her trimmed, I would be worried that it's a clipper burn or other product of the grooming.
You might even find that some dogs get bacterial or fungal infections from dirty clipper blades. Blades should be sterilized between dogs and some groomers just don't do this.
I suggest that you clean this area well with peroxide and then dry it well. You can apply some neosporin and then keep an eye on it.
If you don't see any improvement, you will need to see the vet to rule out an infection that needs medication.
Let me know if you still have questions.
