Not an official medical term but some people describe their anxiety as 'in their head'. They describe how they recognise that their anxiety is mainly in their head and thoughts are going round and round and that they might be 'stuck inside their head and not feeling part of the real world like they do normally'. The obsessive thoughts are pre-occupying them, distracting them and they cant seem to let them go nor ease them or get back to how they used to think before. This is not to be confused with certain psychiatric disorders although many people describe that they 'feel like they're going mad'. One consideration in this is to remember that those who really are 'going mad', are not aware of doing so.
Some of the signs of this type of anxiety is not being able to describe to others whats going on. Searching the internet for answers but not fitting into any desription like anxiety, or depression or anything else. Going about daily life in a fog like anxiety state. Functionning rather well in every day life and looking practically perfectly normal to others in general and not having any OCD or addictions.