What Causes A Painless Lump On The Labia Majora?

hi, i have a small firm movable lump on my labia major i am 23 years of age and i have had it ever since i was a child may be 10 years or more. my mum is a midwife and says its no cause for concern. the lump is painless and has not changed at all. do you think its something serious.