What Causes A Soft Bubble Like Spot On Back Of Head?

So I've had a soft spit on the back top of my head, under the scalp, and there feels like a small bubble or big vein that moves when I touch it. I feel like if I would ever hit my head a bubble would burst it makes me nervous that it's a clot or aneurism. I can feel it when I touch it. It kinda hurts at a dull pain level of 2, when I push on it. I don't wanna push to hard cause I'm afraid it will pop. I'm only 21, overweight, but I'm active and healthy. I couldn't have something like that right?