Thank you for asking the query :
If you had taken the morning after pills only on that occasion, then it will not cause oestrogen excess in the body. Unless otherwise you were on oestrogen supplementation on a daily basis this scenario is less likely.
If you feel that the weight gain is sudden and un expected ( sudden weight gain over a short period of time ) then you should check for your Thyroid function levels. (TFT). A low functioning Thyroid is much more of a common problem that oestrogen excess.
Weight loss unfortunately takes time and effort. And among the available methods, the easiest would be to cut down all the calorie rich foods and do heavy intensity workouts.
Hope this helps. Please feel free to ask follow-up questions via email itself. Thank you.