Sayang, back pain and left-sided abdominal pain masa 13 minggu memang boleh happen, tapi honestly sebab ini involve pregnancy pains kena confirm dengan doctor untuk peace of mind, especially the left-sided pain tu, nak pastikan everything's progressing well. A few of my mum friends experienced similar aches during second trimester (hormones, ligaments stretching, baby growing), tapi yang sebelah kiri specifically kena check in case ada other things yang perlu ditonton. Try rest lebih, avoid heavy lifting, tapi jangan tunda check-up ye, doctor akan tahu best advice untuk your situation. Fingers crossed everything's normal!