My name is ***** ***** I am a small animal vet with many years experience and rest assured I will do my best to answer your question to your satisfaction today.
Actually what you are seeing is quite normal, those structures which look like little bumps at the base of the penis are called the bulbus glandii
They grow in size and then reduce again in response to a dog's sexual urges and are part of what normally happens when a dog has an election.
I know your dog is neutered but even so you will still see them engorged now and again, but in the neutered dog that is all you will see. As I say no cause for concern...
Read more about it here : LINK