Neuter Reaction

My Boxer puppy is almost 6 months old and we just had him neutered about a week and a half ago.  After the surgery the veterinary assistnt advised my boyfriend and I to not let him bounce around and play too much, as she has two boxers herself and the surgery did not affect them in the least bit.  We got him home the night of the surgery and it was just as she said, he acted as if othing had happened and wanted to play. We had a tough time keeping him calmed down. We were given a painkiller for him but did not use it, as he was acting totally normal.  Now a week and a half later, he isn't eating too much, has slight diarrhea every morning for the past few days, acts very shy as if he is afraid of something ( which is VERY different from his normal playful self), and has a small rash on either side of the inside of his back legs. I'm wondering if this has something to do with him being neutered, or something entirely unrelated or if I'm just imagining things  All I know is that my fun-loving puppy is not the same and I'm worried.

[Dan Sims, DVM]

This problem sounds unrelated but an exam should be done.