NO changes of meds & etc. Can do SO good/feel great then HIT with anxiety & physical symptoms for days! Why does this happen?

1st physical checkup. There are so many physical conditions that can cause these symptoms ( many of them easily treated) that you first want to get a physical check up with blood tests that your doctor will order. Once your physical symptoms are stabilized or even sooner if you wish you should have an evaluation by a psychiatrist. A treatment plan should be worked out that may involve medication or therapy or both.
Need more info. What sort of physical symptoms are you having with the anxiety? How do you define your anxiety? When you feel good/great, what is it like for you? How long do these periods of feeling good/great last? People who have such periods of feeling good followed by anxiety for no apparent reason may have an underlying mood disorder. Recommend further discussion with your doctor.