hard lump on head, headaches

About a week ago, I noticed a lump on the back of my head, on the left side, about an inch behind my ear and an inch above my hairline.  It wasn't a gradual thing, but rather appeared all of a sudden that day.  At that time, it was swollen and I also had a headache and neckache (I had been backpacking all day at the time, and wasn't sure if the aches were just because of that, or connected to the lump).  Since then, my neck has been stiff much of the time, and I've had more headaches, even though previously I hadn't had a headache for months.  The lump is not swollen anymore, and it doesn't give me a headache-like pain when I touch it, as it did in the beginning.  Now it's very hard, feels like bone, and doesn't move around.  I'm certain I didn't hit my head or get bitten by anything on my head while backpacking, and I can't figure out what the lump might be from.  Could you give me any advice?  I'm pregnant right now, but I don't imagine that could have anything to do with it...