Thank you for this interesting question. Lumps under your skin can come from many different things and it is important to speak with your doctor
. Of course, on of the most concerning types of lumps are those that are a sign of a serious disease or cancer. These can come when either an infection or a tumor, something that your body recognizes as abnormal, starts to spread. Your lymph nodes will catch what they can and keep it from spreading to other parts of the body. This is a part of a healthy immune response. Most often, these lumps and bumps have simple explanations and resolve by themselves, but if they persist they need to be seen by a doctor to make sure that there is not something worse causing them.
Now, the interesting thing about your question is that the foot is not a common place for many tumors to originate or to spread to. Additionally, there are fewer lymph nodes in this area than in other body parts. Most lumps on the bottom of the foot can therefore be explained by trauma of some sort, either repeated or acute, and perhaps a subsequent infection that is caused. Because you don't know what this is, please speak with your doctor.