Brain tumours do not cause any exterior defects as you're suggesting. The brain is housed in the skull, which is very strong bone, any tumours growing on or in the brain would not cause this bone to buckle. Instead, there would be reduced space between your brain and the skull, causing severe headaches, and depending on where the tumour was; vision alterations, mood swings, fainting, balance and co-ordination problems etc.
If you're very worried, see a doctor! Only they can tell you what the lump is on your head. It could be just "extra" bone, nothing to worry about unless it gets bigger. Or it could be a cyst, which would need sorting out as they can grow. I hope everything works out for you! Good luck!