Marble size growth between head and neck showed up overnight. After Benadryl and rest it went down in size. After any physical activity it swells to double its size. It then goes back to its marble size. It feels like a sack under the skin. I’ve read on cyst and tumors but don’t see anything about growing and shrinking.
I highly suggest you take your dog to the vet and have the growth looked at - don't mess around
Possible parasite issue.
If the issue were allergies, the allergic reaction would manifest in the skin such as itching and dry skin. But because this is a lump it can possibly be an infection like an abscess that has not drained out and therefore has left a lump. You should take her to the vet before it gets worse.
I would recommend you take your pup into your veterinarian to get the lump checked. You were asking about cyst and tumors, from what you're describing it doesn't sound like it. ( They normally don't shrink in size from day to day based on activity. Someone mentioned parasites, which no it doesn't sound like that either. My guess would be either a knot in his muscle or possibly an allergic reaction? But again, I would only be guessing. It is best to have your Veterinarian look at it. Best of luck, let us know what your vet says. Thanks!
I recommend taking pictures of it when it gets larger and when it gets smaller and noting what the pet was doing during either time frame. This way you can show your vet. It could be something like an embedded fox tail which can get nasty fast or even something more serious
