What Causes Headache Whenever You Bend Your Body?

Recently I had a headache when I bent down..It was severe at times and sometimes I was fine..I dont have any problems of sinus or migraine..
What was the reason to have it, I had to take a decongestant in order to relive me of the pain...I wanted to know the basic reason so that will try to avoid if I can..
Thanks for your time Dr.