The condition of lump which you are telling me is worth seeking your utmost attention. If this lump is bothering your pet then you need to take it to vet immediately. It can be growth of cancerous cells. We can not just diagnose this disease unless and until few tests have been made. You need to get its biopsy done where its tissues will be examined to determine if this lump is a tumor or not. If the lump is bleeding or puss is coming out of it then it has more chances towards cancer. It can be simple cyst if it is not bothering your pet at all. Just wait for few days. Hopefully it will be vanished automatically if it is not something serious.