I've been having a knot on my forehead for about 3 years now its not painful but what can cause this? it started off small but now its getting bigger

If : If it is sort of squishy, it could be a lipoma (fatty tumor). You can have your doctor or a dermatologist examine you for an accurate diagnosis.

I : I agree. Hard to tell without a clinical examination but it could be a lipoma or cyst. I would recommend an evaluation by a dermatologist to make sure it is benign. Read more about cysts and lipomas below. I hope that helps.

Cyst or lipoma: without the physical exam it is hard to say. Most likely it is either a cyst or a lipoma. Both of these can get larger over time. See your local surgeon for possible excision.