I am only take an aspirin once a day. why blood on tissue when blowing nose?

Several reasons: It may be as simple as a very dry environment with irritation of the skin inside your nose (mucosa) or irritation due to minor trauma. Nevertheless the only way to make sure you are not dealing with something more serious is to have your doctor look in your nose and maybe obtain blood work to insure your blood clotting mechanism is ok.

Find source of blood: Small amounts of blood occassionally can just be from dried nasal passages, maybe a little more obvious with aspirin, but not caused by the aspirin. Any more bleeding, you need your nose examined.