Yes, this has happened to many people. If you search the forums you will find many similar threads.
The bottom line is that without going to the vet you will never know what it is. In all likelyhood it is a fatty tumor and probably harmless, but it could also be a cyst, infection, or malignant tumor or really anything else under the sun for that matter.
By doing a fine needle aspirate your vet should be able to tell you what is going on. probably around 40 bucks plus the cost of the visit.
A few questions that your vet will ask but I will ask here too...
Is the lump soft or hard? Can it move under the skin or is it firmly attached? How long has it been there? Has it changed in shape or size recently? How old is the snake?