Hello ma am and welcome. The body undergoes numerous changes during pregnancy, and among them the most important ones are changes to the breasts and the uterus. Although you are in the early stages of pregnancy, it could be possibe that your breasts and uterus may have increased in size enough to cause stress to your back. In addition to this, a hormonal surge that is experienced during pregnancy can result in generalized weakness that can also cause the back and neck to feel weak and under pressure. And most important of all, you have reported a fever which could be a sign of some infection (possibly viral) that could be causing the generalized weakness and pain. Hope this helps. Best wishes.