Don't worry hun. About the hair dye thing -I did something similar earlier this year.. was ages washing it out! Your neck will hurt because you've strained the muscles in it, it'll soon get better. You may have gotten some water in your ears which may have affected how you feel in terms of dizziness.
I know how it is to be run down, but it doesn't mean you have no immune system! Like someone said, eating unhealthily won't take away your immune system, your body is more efficient than you think. And 17 is a good, physically strong age! Try to have a chill and not worry. If you have a cold just rest and drink hot drinks. Take a daily multivitamin if you're run down. You'll be okay. God bless.