How is a Lipoma Diagnosed?

I had a lump that grew slowly from early spring last year, until school started. I saw the family md mid december, who said it was a lipoma and I didn't need to do anything... it kept getting bumped, and increasing in size, so after the first of the year, I saw an ortho surgeon, who did an MRI, and when those results came back, I ended up at at Ortho Oncologist, who did a needle biopsy... bottom line????? It's malignant melanom!!! How should the family doctor have diagnosed this?