If it has a small hole with mucous in the center of it then this could be a warble fly larvae also called warbles or wolves. The vet would need to remove the larva as if you try it yourself the larva could get crushed releasing toxins under the skin. Dogs often get them from nosing around animal dens.
If its just a lump no hole in the center it might be a sebaceous cyst. You can see if using a hot compress on the spot will cause it to open and drain.
If it looks like chewing gum stuck to the skin it might be a histiocytoma a benign tumor that is often seen in young dogs and usually self resolves.
However there is a possibility this lump could be a sign of a more serious tumor and as it is growing rapidly you should probably see your vet to get it checked out as cancerous tumors if caught small can often be cured with surgery.
Hope this helps you!
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