I have this brown spot on my stomach.. it was 1 cubic millimeter only 2 weeks ago and it was flat.

Now it's like 2 cubic millimeter and it's not smooth/flat anymore.

in only 2 weeks!

It sounds to me that you need to have that checked by a doctor, I have been told many times that if you have any mole or spot that changes it should be checked out.

Change is the key word! You need to have it checked out!