Sick Rat Terrier Puppy?

My boyfriend and I got a rat terrier puppy about 2 months ago, he's 5 months old and has had episodes where he comes down with hives that we have been treating with children s Benadryl (per his vet's instructions). Recently (about 5 days ago) we noticed a small bump on his back that isn't going away. We have done some research and it seems like his symptoms are similar to that of a dog with a mast cell tumor (the hives are intermittent, shrink and swell in size and he does like to chew on rocks and odd objects symptomatic of anemia in dogs but also symptomatic of puppies in general). This morning he had an upset stomach, threw up in his carrier. We're taking him to the vet tomorrow (they didn't have any openings today) but I'm curious if anyone has experienced anything similar? Also, if you have experience with canine mass cell tumors does this sound like a very aggressive type? I've read online that tumors on the back are indicative of an aggressive, malignant mass cell tumor...any insight would be appreciated, very anxious before the vet visit tomorrow...

I'm sure the vet can tell you much more than me, but it's good that you are going tomorrow. There's nothing really you can do right now, just wait for tomorrow, although I know that's hard. Hopefully it's not too serious! I know this isn't the info you were looking for but worrying about it won't really do any good, the vet will give you the best info tomorrow. Good luck!