I can understand why you're worried, mummy, especially since it reminds you of what happened before.
Lower back aching or mild cramping can actually be quite common around 12 weeks as your uterus grows and your ligaments start stretching. The reassuring part is that you don't have any spotting or bleeding at the moment.
That said, because of your previous miscarriage and the anxiety that comes with it, there's absolutely nothing wrong with calling your gynae if the pain is getting stronger or if you need reassurance before Tuesday. If you develop bleeding, severe cramps, dizziness, or the pain becomes intense, do get checked sooner.
For now, try to rest, stay hydrated, and monitor the symptoms. Many mums experience back aches in the first trimester and go on to have perfectly healthy pregnancies. Hoping Tuesday's scan brings you some peace of mind 🤍