Maggie had trouble adjusting to food changes too. She had diahrria for about a week once, but that was due to rawhide chews she was eating. If she has too many treats in a day she has diahrria (I can NEVER spell that word) for a day afterwards. But I notice that as she gets older, she's having less trouble with changes in her diet, gets wet stools less often, regardless of treats and whatever toys she's digested
I notice that she'll have very loose stools if I even change the type of treats I give her. Others may have other experiences, but that's mine. I have a friend who's had a similar problem with his puppy (non boxer) and has been having problems for about a month.
Food allergies are always a consideration as well. I notice Maggie doesn't do well with any sort of Liver product. Gives her bad diahrria. But she does well with chicken or lamb.
I feed her Nutro Max Puppy formula. I switched her a couple of times before we settled on this and now she does very well (and she likes it).
Good luck. Go back to the vet if it doesn't improve in a few more days or if she's exhibiting any other signs of illness such as not eating or drinking or drinking excessively (much more than normal), lethargy, etc.
My personal opinion is that puppies must have very sensitive tummies and until they mature some and get accustomed to food that hasn't come "pre-digested" from its mother, they are gonna have some tummy problems. After all if you think about it, it was only a matter of WEEKS before you got the pup that he was weaned from its mother's milk.