Sayang, belakang sakit memang common masa pregnant, tapi kalau menekan-menekan dan sakit terus-menerus macam tu, better jangan tunggu, go see your doctor or midwife ASAP ya, especially since you're first time mom. Dia boleh check what's going on and pastikan baby okay. Few of my mum friends juga experienced contractions yang felt like back pain, so ada possibilities, tapi doctor je boleh confirm. Take care!