It happens - blowing your nose too hard can temporarily cause the blood vessels to constrict causing transient hypoxia - lack of oxygen for a short space of time. I doubt it would make you black out, more likely cause you the light-headed feeling you had and perhaps a little unstable on your feet. It's probably nothing serious if it is only occurring when you blow your nose - some people get it when they sneeze or if they have a tantrum because when the blood vessels narrow, your blood pressure will raise temporarily. If it especially troublesome and you are overly concerned, I suggest going to your doctor and having your BP checked. Hope this helps.
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Yes, a strong nose blow can trigger a vasovagal attack and make you feel dizzy. There is nothing much you can do about it except take fluids and eat healthy. Also sinusitis can infect ear which can also cause dizziness. Hence consult an ENT doctor to rule this out.
You could also be having EBV or infectious mononucleosis and this needs to be ruled out by blood tests. Please consult your PCP for primary examination followed by proper referral. Take care!
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