Hello,first don't try to jump on conclusions without proper diagnosis ,to give yourself unnecessary stress.bleeding from cheeks could even happen from a deep cheek bite.but if it is continuous from long then please visit a dentist and get yourself checked.
Yellow colouration of phlegm is normal but if it contains blood surely you need a checkup.visit any good ENT surgeon
As per your smoking thing you haven't mentioned the time onset of your habit.cancer even happens in vegetarian ,non smokers,non alcoholic people.so yes smoking is a inducer but not a sure initiator for cancer.
Next Steps
visit your dentist and a ENT surgeon
Health Tips
as smoking is not a good habit try to stop it.
best way is to reduce it one by one per day or set goal for yourself