It is not just lung cancer Alex, there could be other problems like lungs or something heart related. I think it is high time that you consider for a doc visit and do not postpone it. Because any of the condition shall require timely treatment and will lead to unnecessary complications if left untreated.
i am sorry to ask this but are you a little over weight this can happen also if you are asmoker it carnt help if i were you i would see your gp very soon this will help you to alleviate your worries and if there is any thing to be upset about you can get it sorted good luck to