Thanks for the additional info!
There are a great number of things that can cause a lump on a dog. The good news is that most of them are not anything serious.
The most common reason to see a lump on a Golden Retriever is a fat lump called a lipoma. We don't know why they come up and they are nothing at all to worry about.
Next, it could be a bruise or a seroma (fluid filled pocket) although in this location, this would be a little unusual.
It could be a reaction to an insect sting.
In the area you have described, this could be an umbilical hernia. However, these are usually present from birth and it would be unusual to just notice it now. It is however, possible that there has always been a very small hole and now some abdominal fat has poked through and herniated. If this is the case, your vet will be able to tell right away and can probably push the fat back in. Again, this is nothing to worry about.
And then the final possibility is the one that we are most worried about and that is cancer. Most cancers (but not all) are not painful to touch and usually look irregular in shape and often dark purple in colour. However there are many different types of cancers and not all fit this description.
If your vet is worried that it may be something serious, they will do a fine needle biopsy which is a very simple and mostly painless procedure in which they will take a few cells from the lump. They may be able to determine in the office what it is, or they may send it away to have a lab test it.
I hope everything goes ok!
I hope this information helps!