Hello
Welcome to health care magic.
In post partum period,if you are exclusively breast feeding the baby,there are less chances of you getting pregnant due to
contraceptive effect of
lactation.
But you said you have started getting periods.So the contraceptive effect of lactation will not be that strong.You can go for testing HCG levels in blood for confirming
pregnancy if present.
Dark colour of nipple is due to previous pregnancy.
Lower back pain and nausea can be due to
pelvic inflammatory disease or
urinary tract infection.
PID can result in delaying periods.
It can be confirmed through vaginal swab microscopy and urinanalysis.
Accordingly,antibiotic course for 7-10 days willl be given to cover up infection.
Maintain adequate intake of water.
Use soap and water for genital hygiene.
Meet your gynaecologist in this regard.Also take advice regarding type of contraceptive to be used in postpartum period.Implant or Injection are the best choice under such situation.
Hope this information helps.
Thanks