Last week I started taking a women's one a day vitamin. Every since I have had bad tension headaches. Is this coincidence? I take Zyrtec (cetirizine) and Flonase

No correlation. has more to do with your sinus issues, that can create a tension headache. Taking your Zyrtec (cetirizine) or Flonase should help. Make sure you are hydrating well as the first sign of dehydration is having a headache.
Yes, coincidence. While you didn't state why you're taking Zyrtec (cetirizine) & Flonase, sinus congestion could cause your headaches.