The only one who can diagnose you is a doctor, so that is the route to go.
Read my IBS 101 post for diagnoses.
Its not likely cancer at your age. There are many other things that could be causing the symptoms.
" Oh and could the fact that I do literally no exercise and bad diet have an affect?
That can have an effect no matter what the problem is overall.
You need to see a doctor and don't worry to much, but that is the first step and you shouldn't put it off.