This sounds like an allergy since the vet couldn't find other physical causes.
It's usually a food allergy, but it must also be considered whether or not the dog is exposed to environmental toxins such as cigarette smoke, insecticides (including flea products), carpet cleaners, even the detergents you might use in your laundry (dogs are known for cuddling up on our beds and up against our clothes right?).
Allergies to foods can manifest with what we typically recognize such as sneezing, coughing, wheezing, but also with intestinal disturbances and skin disorders (mostly itching). Yes, foods can be tolerated for significant lengths of time without causing problems and then suddenly begin to cause symptoms.
Sometimes, treatment with corticosteroids, antihistamines and other prescribed medications are helpful in getting the episode under control, but for long-term use, it's not always indicated. A good relationship with your medical caregiver is important in order to establish discussions about treatments.
This is an important link to explore possible allergies and allergic reactions
Changing foods is something you can try right away. Give the new food at least a week to determine whether or not it makes a difference.
Many people have found a successful solution when they provide only home cooked meals, however, this is not recommended for more than a month or two since it's nearly impossible to provide a nutritionally complete homemade diet.
The typical combination in this diet is two parts starch (rice, potatoes) and one part protein such as soy, salmon (also provides essential fatty acids), venison, rabbit or duck.
There is much more discussion about feeding options and procedure here http://allergies.about.com/cs/canine/a/aa091800a.htm
To help with the sneezing right away, you can install a vaporizer without meds in it to help keep her nasal passages moist and clear. The blood is probably from superficial irritations from the sneezing. Plain saline nasal spray (if tolerated) may also help, but you must be sure it's got no medications in it - just plain saline spray.
Providing a heat source may also be helpful - a heating pad set on low under a thick blanket - making sure she's not even aware of the wires (check often for any biting and remove immediately if she does).
Other heat sources can include 'rice socks'. Just take a thick regular sock and fill 3/4 with raw rice and knot the end. Microwave it 1 to 1 1/2 minutes, shake it through to distribute the heat and lay them in her sleeping area (she'll put them where she needs them on her body).
Hopefully it will self-resolve with the food options listed above and the environmental modifications (vaporizer/heat) - it could be in as little as a couple days, but give it a few weeks anyway.