I have a small lump that i can feel with my tongue on d roof of my mouth. my dentist said it was nothing what cud it b? it's been there few weeks now

Hard to tell: It is hard to tell without more information... Could be a torus. If it just showed up and does not go away in two weeks i would suggest another opinion.. Or at least a diagnosis.

Lumps in the mouth: Most 'lumps' in the mouth are the bony prominences of the internal skull. Many people aren't always aware of them and then get nervous when they feel something hard. I would trust your dentist. They would not steer you wrong.