How severe is this pain? Back pain is extremely normal, and almost every female has it, especially in later pregnancies. If it's severe and/or excruciating then you need to be looked at, but mild back pain is usually not a cause for concern. Try soaking in a bath of warm (NOT HOT) water to see if it helps relieve some of the pain.
I'm 5 weeks tomorrow and my back has been killing me all week. It's usually pretty normal!