You could ask your vet to check for a potential health issue next time you are in-- but I don;t think this is likely to be due to pain or health in such a young puppy
and do please put the word 'dominant' out of your mind
-- especially with a puppy that is only a baby still -- true 'dominant' behaviour is very very rare; almost everything that is (unfortunately) described and often, disastrously treated as 'dominance' is very common, basic dog behaviours. Or to put it this way -- if a toddler snatched a toy from another, or argued with a grownup, would anyone call that 'dominant'? Of course not -- it is behaviour totally expected and fully in line with both age and the fact that the child has not matured enough to do otherwise. Dogs are the same.
If this is guarding behaviour -- EXTREMELY common and NOT dominance
-- it is however indeed something that needs to be tackled immediately (as an owner training responsibility; it isn't a behaviour or personality problem with an untrained puppy who knowns no better). Your pup is at a very critical age for learning the right behaviours and if guarding begins it can become frustrating and even dangerous. The way to manage it is to train your dog not to be bothered at all by being asked to move, give up a toy, chew, food etc. These are once again all owner responsibilities though (I am going through all this basic training right now too with puppies!) -- a pup/dog that has never been trained kindly and in a fun positive way (never with punishment) to behave as we would wish will simply have no idea what its owner expects and this can add to an increasingly unhappy and potentially fear aggressive, confused dog (though not a 'dominant' dog).
There are very basic techniques for managing this -- do download Dr Ian Dunbar's free book After You Get Your Puppy (see link below). There are also loads of training sheets and guidance at the links pinned in the training forum and all will have information on the basics of puppy training and managing guarding behaviour. There are also many earlier threads on this topic with links to good advice -- so many things you can do. Also your up is at a critical age for puppy socialising classes and a good trainer wil be able to give you fun activities to help address this issue (look for a trainer with CPDT or APDT qualification).
Ian Dunbar book:
www.dogstardaily.com/free-downloads
The good news is that this is very easy behaviour to train away from at your puppy's young age.