Hello
There could be various reasons for weight gain Like- Hypothyroidism that is less functioning thyroid , Diabetes , imbalance in female sex hormone , intake of oral contraceptive pills, familial predominance, anxiety.
In your case you have a recent change in your period habits which can be due to the changes in female sex hormone and very much likely responsible for the weight gain.
About the pain you are getting it could well be gastric ulcer disease which has a typical history of pain after food.You can start some antacids like omez if the pain subsides it confirms the diagnosis, or you may have to undergo gastroscopy to confirm or exclude it.
Various tests like thyroid function, glucose levels, cholesterol levels,sex hormone assays may be required.A gynecologist will be the right person to see.
Hope this helps you.
Regards
Dr.Tharun
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