Thanks for your query.
After taking emergency contraceptives, there is always some amount of withdrawal bleeding due to the hormones effect.
Hence, what you experienced on 18th was the withdrawal bleed.
Also, emergency contraceptives will prepone or postpone your normal periods by a week or more.
Hence, the 25th was likely your period.
the next menstrual cycle is generally not affected by the emergency pills.
it could be due to stress
, thyroid hormone imbalance
Wait out a month, and if this persists next month too, then consult a gynecologist.