I've had headaches since the end of my tww. I take Tylenol and they go away. If Tylenol isn't helping, then you need to let your RE know. It can certainly be a sign or pregnancy due to changing hormones, but as mentioned above, it can also be an indicator of dehydration.
Headaches are a sign of pregnancy, I have had them with every one. They also are caused by hormones though and sometimes this will throw you off. 5dpo is pretty early for preg headaches though cause there isn't really and HCG in your system until implantation.
Headaches can be caused by many things. Fluctuating hormones can cause headaches, dehydration can cause headaches, as well as stress like you mentioned, and many other things. I'm not sure if headache is a sign of pregnancy or not. I've only been pregnant one time, and that was almost 17 years ago and I do not recall having headaches. Good luck!