Continual Cramping & Back Pain

Clare • Age 26. Married. 👦🏼😇😇😇🤰🏻
Today I saw my OB and had a sono to check baby's size. Everything looks great, baby is low and I am 2cm and 50% at 36+6. I'd been having some period-like cramping and back pain that came and went earlier in the day, but tonight starting around 10pm to now (5:30am) I've had constant, continual, and more intense cramping and back pain. I had to get in the bath, which helped relieve some of the pain, but I have not experienced much relief overall. These are not pains I can time, as they are constant, and I don't know if I'm having contractions or not 'cause my belly seems to always be hard and compact. Any insight into what is going on? Is my body simply preparing or could this be labor starting?