As scarolingari pointed out, EDs and low BMI can mess with your cycle. But the cycle is not nearly as stable or reliable as many people imagine. Even otherwise healthy women often cycle irregularly. Just keep track, talk to your PCP and/or ED treatment team, and get comfortable with the idea that your "monthly cycle" is in reality not necessarily monthly or a predictable cycle.
the ED will definitely mess with cycle, it does mine, ive almost skipped, and have been as much as 2-3 weeks late. and your bmi is alot lower than mine is, ed just throws the whole system off.