WOW! I cannot believe someone else has had this problem! I have SEARCHED online for a year about this. Millie has the same spot, and does it the same amount. She will go months without doing it.
So let me tell you, calm down. If it's like Millie's case, it's not her health. I have taken her to 3 vets and they have all said she is fine. I was so frustrated and convinced she had something terribly wrong with her. We figured out what happened about 3 months ago. The second harness she ever had, we bought her a retractable leash to go with it. When we walked her and we locked it, I guess it would pull on her slightly and scare her- never hard enough to actually hurt her. But she is only 3 lbs and a sensitive dog, so that was enough to make her scared of certain movements down there. Got her a new leash and it stopped (this is when she was doing it everyday). Now she still does it but only if I forget and pick her up a little quicker. There were times when she did it when there was NO WAY I had hurt her. Just brushed over her skin...but it's enough to make a dog scared if they are sensitive! Now I feel awful because I feel like I have scarred her for life and she will always have fear sometimes and not be as trusting. Sometimes I can be more forceful and poke all over her body and it doesn't bother her- but if I "sneak" up on the movement, that's when she yelps and takes off running.
I would still get your babe checked out as I did, but I will say- for right now breathe because I went/am still going through the same thing! Dogs are funny little animals!
Millie loves me with all her heart and trusts me with everything, but sometimes her sensitivity gets the best of her!
Let me know how everything goes! I would love to see what the vet says because my vet didn't have a clue! He almost made it out to be that it was my imagination because they couldn't find anything wrong with her.