Skin conditions are common. They vary between minor and transient rashes to serious conditions like skin cancer. Given your concern, I would strongly recommend that you see a dermatologist
(a skin specialist).
To answer your question--it is possible for skin rashes to form into lumps. The most common form of this is something known as keloid. Keloid is an inflammatory reaction that can occur to any damage to the skin. For example, if there is a cut or rash, inflammation can form. In some people this inflammation is exaggerated and forms a fibrous scar. These are more common in dark colored skin such as African Americans, but this can occur in anyone. These can be removed.
Other rases can form lumps. A key one to consider is a palpable (bumpy) purpura (small dots). This is often caused by vasculitis
or vessel inflammtion. There are many others.
Finally, rashes are often from inflammation. Inflammation can result in a different environment that can lead to alterations of cell growth. Therefore cancer can form. Squamous cell cancer or melanomas are possible.
I think you need to see a dermatologist. Skin conditions are difficult to diagnose without seeing and therefore going to a doctor
would be of benefit.