Have you always had nosebleeds? If not then let your doctor know, especially if they don't clot right away. If so then look below.

I have and typically its caused because the veins in the nostrils are too close to the surface. Nosebleeds are greatly exacerbated in low humidity conditions which can be caused my HVAC or dry environmental conditions, cold weather, dry heat etc.. A cold type humidifer will help as well as rubbing a light coat of vasoline on the inside of the nostrils. Other moisterizers may work better, I personally don't use them because they also interfere with sense of smell. I put up with the nosebleeds...