I'm 24 and I smoke, I have a bad cough and blocked nose , my doctor says it's a chest infection and gave me antibiotics. I'm just worried its lung cancer or something mre sinister. Help

as you are young & doctor confirms it as a chest infection, more likely it is. But smoking gives different result in different body, be careful,
antibiotic will help you to get rid of the infected bacteria if correctly used, and you can add other relieving medications, like brochodialater , mucolytic, anti histamine and don't sleep flat, make your bed at 45 degree propup. Inhale steam with deep breath like you are excersicing your chest for > 10 min 2 or 3 times a day. Humidify the room. If these won't help in 1 or 2 days go to a hospital, you may need to see with chest imaging like X Ray.
quit smoking,
do your family memebers have wheezing or Asthma or runny nose sneezing or alergy ? If yes you have a potential to become a Asthmatic & smoking aggravate it.

If you had lung cancer from just having a congested chest you'd be in the medical journals for sure. You just have a bad chest cold. Take the antibiotics, get rest and use a steam inhaler or just inhale Vicks in a steam kettle several times a day to help with the congestion.

Be sure to go back to the doctor if your symptoms don't go away after 2 weeks. You could have your chest congestion turn into bronchitis very easily. Try to quit smoking while you are ill also. This puts quite a strain on your lungs when you are sick with any kind of lung infection. Try to quit altogether after you are well. It's the first 2 weeks of not smoking that are the hardest. I ended up having bronchitis for 9 months - yes NINE months. I quit smoking. I even had to quit working in an office where other people smoked in order to get rid of the bronchitis. I kept going back to my doctor until I was better. Nine months to have bronchitis was way too long. Hope you don't make the mistake I did by still keep smoking or hanging around other people who smoked until I got better.

If you decide you'd like to give up smoking, get the patches. That is
How I quit... I needed the patches but I'm smoke free for a number of years... if I ever get the urge, I drink a glass of water and I also learned, the feeling passes in just a minute or two. Good luck.
If your nose is painful, you may have a sinus infection. An ENT would be the best source in that case, but talk to your doctor first.