What Causes Nose Bleeds In A Breast Cancer Patient?

I have had a low white blood cell count for approx. the last 5 or 6 years. I have seen a doctor who orders regular blood work. The count goes slightly up and slightly down with each visit. The Dr. says it could be caused by medication I m taking, and as long as it s fluctuating and not on a continual downward spiral, there isn t much concern. He scheduled me for a bone marrow test but the day I went in, my blood count was up a little so he didn t do it. The last time I had blood work done, (Dec. 2015) my white blood count was 3.4. I had radiation therapy for a very early stage breast cancer following a lumpechtomy two years ago. No chemo. For approximately the last month I get an occasional (maybe once a week) nose bleed which I haven t had in years. It usually happens after I blow my nose even though I don t blow very hard. Today I was shopping and it started to bleed for no reason. I can stop it by stuffing a tissue in my nose. It doesn t last very long. It bleeds from the right nostril. My blood pressure is under control through medication. My meds are: lisinopril, tryamterine HCTZ, lovastatin, ecitalopram and letrozol. So, my question to you is what could be causing my nose bleeds? I am a retired female, age 72.