Does plan b cause frequent urination ?

No: this is the short answer. Now I think it is important to get the urine checked for infection.

Plan B: Plan B and other hormonal emergency contraceptives do not include frequent urination as a side effect (http://www.drugs.com/sfx/plan-b-side-effects.html). However, post-coital bladder infections are relatively common and could explain frequent but usually painful urination. Go see your Family Doc, GYN or Planned Parenthood for evaluation & appropriate treatment.