Experiencing light cramping in the lower back during week 5 of pregnancy is generally considered normal. It can be caused by the uterus stretching and growing, as well as hormonal changes. However, if the cramping becomes severe or is accompanied by heavy bleeding or other concerning symptoms, it's important to consult with your healthcare provider to rule out any potential complications. Otherwise, try to rest, stay hydrated, and practice gentle exercises or stretches to alleviate discomfort.