This exact same thing happened to me a couple months ago...lump on my head, don't remember ever hitting it, etc.. I asked my doctor about it when I was at her office for an unrelated reason and she felt it and said that sometimes these bumps will form but they are not worrisome unless they start growing. So she told me to just let her know if it gets bigger but not too worry about it. I also experienced the blurry vision like you said, but I ONLY experienced it once I started worrying about about the lump and it would go away when my mind was distracted with something else. So basically I was imagining the symptoms due to anxiety!
Therefore I don't think you have anything to worry about, just make sure it doesn't grow. And it doesn't hurt mentioning it next time you are at the doctor, just for the peace of mind, but it is most likely is nothing!