Is it normal to menstruate every 2 weeks?

No: Menstruation is not called "normal" if it happens more often then every 21 days (3 weeks). One's Ob-Gyn doctor can check to see what is the problem. Maybe menstruation is every 4 weeks, with vaginal bleeding due to other causes at the 2 week mark during each cycle such that it looks like menstruation is happening every 2 weeks. One's doctor can figure this out.