I'm 5 weeks tomorrow and my back has been killing me all week. It's usually pretty normal!
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.