Thank you for your query.
Firstly, after childbirth, and specially during lactation
, it is common to experience irregular periods
Sore nipples also are common while breastfeeding
, specially during lactation, predisposes to lower back pain.
Bending and abnormal posturing which is involved in raising a one year old often puts strain on the tummy and back.
These are not confirmatory of pregnancy.
Since your periods are irregular, you would need to see a gynecologist to rule out pregnancy.