Why does Hitting my head mess with me so bad

I hit my head a couple months ago, I'm really clumsy so this is something I do regularly, I started to worry me so I went to the doctor and they told me I was fine and that hitting my head like that caused a really bad migraine. But I don't understand because when I hit my head it took me down and I lost my vision. I hit my head yet again yesterday and it made me throw up and I got light headed and everything but I'm fine now, there's not a dent or a knot in my head and they said to only worry if there's a dent. So if im just hitting my head and it's no big deal then how come any blow to my head takes me the hell out??? My doctor acted like I was being dramatic so I don't want to make another trip to him because I hit my head AGAIN