Autism, as I am sure you are already aware Twilight, is a disorder which can include many different symptoms, and has a broad range of severity.
Anxiety, however is not uncommon to a number of those with the disorder. It can be caused by any number of things, depending on the individual involved. Someone might get over anxious regarding social-interaction, someone else with a change to routine, crowds, animals, their job...there is any number of reasons as I have said.
The effects can also differ and effect you physically,... headache, nausea, sweating etc and also psychologically... lack of concentration, sleep disruption or obsessing etc.
A common factor to many with autism, is serotonin levels. When serotonin is reduced in the amygdala,[ a region in the subcortical brain that processes fear and anxiety] it`s activity increases, which in turn leads to more fear processing and can cause anxiety and depression.
However, whatever the cause, anxiety is unpleasant.
Do you keep note of those things and instances that cause you to be anxious Twilight? Sometimes it helps if you keep a record. By knowing the things causing you, yourself, to be most anxious can be a good way for you with your doctor or health carer to find positive ways to help deal with it.