Should i go see a doctor or am i worrying over nothing?!

I've had headaches on and off for a couple of months now they range between a dull aching at the front of my head which dont go away to stabbing pains in the back of my head. They also seem to get worse when i stand up/bend down for a couple of minutes then they calm down a bit. Im also always tired no matter how much sleep i get and i havnt really felt myself for a while. I become more annoyed/down about stupid things but that could just being a teenager. Ive tried drinking between 1.5 - 2 litres of water a day so its not that im dehydrated. ive also notices that i am becoming ill more regularly like i have more colds, sore throats, feeling sick. Im just not too sure what it could be should i go see a doctor? thankyou x x

Also does anyone know how to get rid of a chesty cough?