There are several possible explanations for how you are feeling right now. The first is that pregnancy is slim, as you mentioned, but post-vasectomy, the rate is still something, about 1% chance of pregnancy, and that number is XXXXX in the first six months after the procedure. This is something to consider if your period comes late.
The other possibilities related to nutrition/dehydration or endocrine abnormalities. If you haven't had enough to eat or drink, your blood sugar could be very low and give you these feelings or dehydration could case the same - this would be unlikely to cause a chronic problem, however it is possible if your eating or activity habits allow this to happen. There is the possibility that you may have an endocrine abnormality, most commonly a thyroid problem, and so the first step in this evaluation would be to measure your thyroid function which is a very easy test for your doctor to send. Other possibilities related to increased generation of insulin (which could cause the same low blood sugar as I mentioned above) that could happen by a part of your pancreas that makes more insulin than it should. There are other types of neuroendocrine entitities that make extra hormone that can cause you to feel this was, i.e. epinephrine secreting.
The best thing to do is to have this evaluated by your doctor who can evaluate your status and determine whether there are any hormones that are not regulated properly. The fact that you are not having any abdominal pain makes this sound like it is not an emergency, however if you do feel like you need to pass out or do pass out, or develop abdominal pain, I would seek medical attention right away.
Meanwhile, here are some options for reducing your nausea:
Drinking fluids is always a good way to help nausea and there are several medications that are effective against nausea including phenothiazines (like Compazine and Phenergan), 5-HT3 antagonists (like Zofran), dopamine receptor antagonists (like Reglan), antihistamines (like Benadryl), and anticholinergics (like Scopolamine). I am attaching an information sheet on nausea and vomiting that may provide some more information: http://www.gi.org/patients/gihealth/nausea.asp. Many say that ginger is effective against nausea in certain situations. I am attaching a link with information on this: http://www.umm.edu/altmed/articles/ginger-000246.htm. An additional option is something that is highly recommended by many women during pregnancy who take this for morning sickness called preggie pops. You can read about them at http://www.threelollies.com/
I hope this is helpful and please let me know if I can answer any more questions.