Re-post bout puppy issues

Question
Hello again, I have asked you a question before bout my pup and have some new developments and am curious what you think about the siutation. I have the 7 month old terrier mix, male, neutered, up to date on vet care puppy. He is great and very healthy. However, ever since I got him, at 8 weeks old, every once and a while he would turn around and suddenly look at his bottom area, sometimes he would then yelp while others he would just go back to his normal activity. This happens with no notice, out of the blue, and seems to occur mostly after a new or stressful event (but it has happened in normal situations like simply playing). I have recently taken him back to the vet because the behavior is getting more frequent. The vet did a complete exam, tested his joints and skeletal system and made sure his anal glands were not full. All was completely normal and he was called very healthy. However, the vet wanted to try a few meds just in case to hopefully put an end to the odd behavior. He started him on flagy in cause he had a parasite like giardia or a bowel inflammation problem (did not work), now he is on pain meds called deramaxx just in case he has some unseen arthritis issues or orthopedic problems (has not worked so far), next I believe he is going to try psychological meds. After that history I have two questions:  

1. If my pup has muscle spasms would the deramaxx take care of that? My pup is very strong and loves to run and jump so I thought maybe the reaction was to a muscle spasm. Are muscle spasms common and if he had them would the deramaxx take care of them?  

2. Is it common for a puppy to have such an odd and extreme behavior as reacting to his bottom for psychological reasons? If it is psychological is this something he was born with or something he developed?   I try my best to get him the best care possible and make sure to keep him up to date on vet care, heartworm treatment, and flea prevention so this odd behavior is frustrating that no one can find a reason for it. He has seen 2 different vet offices and no one can say a reason. He has been medically cleared as far as I know but he continues to react in this odd way. Any insights would be great.

Answer
Rachel,
While I do remember this question from before, if you have had him to two other vets and they cannot find any reason for this behavior then it is very possible it is a behavioral issue.
Whether or not he was born with this or it's some thing he has acquired is not something I can determine.

I really doubt that these are muscle spasms. Duramaxx is a pain reliever, but it is not a muscle relaxer. I don't think he is having muscle spasms anyway.

I think he feels something, be it a muscle twitch or a nerve tingle (more likely) or he could be having a petite mal seizure even. I would ask the vet about that too. Petite mal seizures are small, sometime significant motions that are sometimes frozen moments when a dog looks like they are staring into space, or a lip twitches, or even looking around oddly. Some dogs stare at walls, etc. So this could very well be a form of a petite mal seizure. It is at least worth mentioning to the vet.

Other than that, I am afraid I have no other ideas. You might want to re-post this to one of the veterinarians in here.
I hope you find the answer soon and he gets some relief. Please let me know what you find out finally.