This is a very common problem among smokers. When you smoke, your blood vessels shrink and stop bleeding even when the gums are unhealthy. It is not a good sign and shows that your gum disease is progressing without showing any signs. So it is hard to detect.
Once you quit smoking, your blood vessels become normal and disease signs appear. You need to get a scaling done and also get your gum health assessed with a qualified dentist.
It is probably due to plaque and tartar deposits and will get healed as you go for deep cleaning.