Thanks for the query.
If you are breast feeding the baby, there could be some amount of hormonal fluctuation that can lead to irregular menstruation
And the hormonal effect can lead to symptoms like pain abdomen, nausea etc.
If you are not using any contraceptive
method, better to rule out pregnancy once.
If the pregnancy test comes positive, go for ultrasound
to estimate the gestational age
and proceed further.
If the test comes negative, consult gynecologist get examined and take symptomatic treatment.
You have to continue iron and calcium supplements
during lactating period also as the need will be more during this period.