Thank you for the query.
Most likely you this lumps are calluses
. This condition is caused by frequent compression of one part of the skin. Due to such process, skin cells are migrating to this place and cause much thicker part of the skin. It is some kind of protection against the compression.
First step in treatment of this lumps are comfortable shoes and soft shoes inserts so this areas wont be compressed anymore.
In treatment of it you can also try salicylic acid
medicine applied on this areas. Some doctors may suggest you surgical removal of it, but this is not a good idea as foot wounds are very hard to heal.
Hope this will help.