The main culprits of postpartum back pain are:

• Car seats that are heavy and that you carry with unusual movements.
• Strollers that have to be lifted and folded.
• Picking up the baby in his or her crib, which requires bending at the waist.
• Breastfeeding in a position that does not support the lower back.
• Carry the baby in your arms for several hours a day.

Back pain usually manifests itself as a dull ache in the lower back, a sharp pain in the lower spine or where the pelvis meets the sacroiliac joints (connection between the spine and pelvis), says M. me Davey. It also happens that new mothers suffer from sciatica or neuralgic pain that radiates down the leg.

Source:

Sunlife.ca, https://www.sunlife.ca/en/tools-and-resources/health-and-wellness/preventing-and-treating-illness/protect-your-back-after-having-baby/