Backache and period pain

I’ve woken up in the middle of the night to sharp backache, now it’s just a dull period pain - it doesn’t come in waves like you would think a contraction would.

Baby was 4/5 engaged a week ago and my body has had a ‘clear out’ this last week. This is my first sign of uterine pain... I’m wondering if this is the start?! 😬