large painful lump underneath substantial birthmark

Hi there.

I've got a mole on the inside of my left arm, something like 4 inches long and 2 wide, with a thickness of a couple of centimetres.
I've been told recently by a dermatologist that there's little risk of it turning cancerous.

Nonetheless I've noticed in recent days, strangely a couple of weeks after the examination I had for it, that there is a large, quite solid growth directly underneath the birthmark. The birthmark itself hasn't really changed in colour or shape, beyond the fact that it sticks out a bit more now due to the lump.

It does seem to be attached to the mole, and I can't possibly believe that this lump is unrelated to the mole, given its precise situation under it. It is a bit painfull when I touch it, or bend my arm or when any pressure is applied to it.

I wasn't aware that malignant tumors could grow as fast as this, but I'm still worried that perhaps due to extra proding and examination recently, it's taken a bad turn.

What could this lump be, and is it serious?

Thankyou!

[allmymarbles]

Go back to your dermatologist. It could be nothing at all - an infection for instance - but you want to be sure.