Oh OP, I think your fears are quite normal, I certainly convinced myself I was having a miscarriage/going into very early labour multiple times.
There could be loads of reasons as to why baby is measuring a few days behind, and it may be that they catch up, or they’re just born a bit on the smaller side. The midwife would have been able to see that baby’s heart was beating, and if it wasn’t she would have told you.
The lower back pain can be debilitating at times, but is also very normal. All of your muscles and ligaments are adjusting to this new weight inside you. The cause of the cramps may vary - I had horrible pains all round the sides of my lower tummy, exactly like period pains, and I was terrified. It was just my womb stretching to adjust for the baby, hence why it can feel very similar to a period cramp.
You said it yourself, the baby was fine at your scan. There is no reason to believe that anything has changed in the meantime. The little angels give us so much cause to fret and worry, I know, but what you’re experiencing is normal, even the concerns.
I’d have a chat with the GP to talk about how it’s making you feel, and the fact you’re not able to go about your day to day without feeling this anxiety and stress. Your next scan, whether it be NHS or private, will show you there’s nothing to be scared of, I’m sure. X