What's happening? Help!

Before I start, this is my second pregnancy and it's been so different to my first that I don't really know whats happening. At 8:45pm I got sudden lower back pain (right side) that has mostly been constant but occasionally dulls and just lingers as an ache for a few minutes it's now 12:33am. My labour started with my first child in my back but I really can't remember what it felt like. Im only 29+1 but with a Low-lying placenta I don't know what to expect to happen. What do I do? Any advice would be fab x