If you habitually smoke weed, and over a long period of time (I know this isn't you right now, but it has been proven) you CAN develop lung cancer, it does mess with seretonin levels in your brain, lower your sperm count, and you can develop an addiction, among other things.
That being said, smoking pot once or twice a week isn't really a big deal (from what I've been told by doctors yes, shocking I know, but some doctors will actually tell you all about the pros and cons of pot), but you are kind of young so yes, it could have an effect on your brain growth even though you don't smoke a lot, because your brain is still developing and creating neuropathways and whatnot.
I think however your question of how you're feeling, the headaches, dizziness, generally feeling shitty, that's probably low blood sugar and dehydration. Is it usually in the morning when you're not feeling so hot and then about lunch time after some lunch and whatnot you feel better? I'd say just watch what you eat and if you get mad munchies, just eat better stuff and try not to eat up all the junk food in the house.
