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It is common to have menstrual irregularities for a few months while breast feeding as breast feeding is known to interfere with hormonal
balance needed for regular menstrual cycle. Further the back pain
and stomach pain you describe seem like symptoms of premenstrual symptoms.
Cyclenorm e and p contains hormonal drugs which may be secreted in the breast milk. It would not be wise to use it unnecessarily. Hence I encourage you to visit your gynecologist for evaluation. You may start the drug under his or her guidance.
Hope this answers your query.
Wish you good health.