Working in an office with any kind of pain is not fun...
Don't be embarrassed to bring what you need to look after yourself into the office. Tell your employer you have a chronic illness, they need to know and shouldn't treat you any differently because of it.
Your employer has a duty of care to you and if you need to bring in a hot water bottle or heat pack then go ahead and do it. You are showing you are willing to work despite your illness and they should understand that.
It is unlikely that your manager will look further into your condition after you tell them what is wrong with you. However, if you describe your main symptoms and that they fluctuate, your manager will be able to take note.
It is usually your HR manager that will be responsible for keeping an eye on your well being, so, if you feel your manager is dismissive, go to HR and explain your situation.
Take a break!
You will need to stretch and move around, there is nothing wrong with doing so. Make sure you take your lunch break and do something with it. Don't eat lunch at your desk!
Amazon Prime Items that may help
Disclaimer, these are purely recommendations I am not qualified to give medical advice. Do your research and consult a medical professional. Click on the pictures to be redirected to amazon.