Are you getting to bed at a reasonable time? How about going to bed a bit early? Sometimes our bodies are stressed, and it is a good idea to make sure your body is getting the rest it needs.
Also take a look at what you put into your body. How you eat directly relates to it's performance including how your brain functions. Sugar in all it's forms (high fructose corn syrup, white bread, white rice, ect.) has an effect that makes your body sort of hyper active for a brief period of time and then abruptly the energy source is burned out and this results in a crash.
If you have eliminated stress, lack of sleep and too much sugar then I would look at the medication, especially if you did not have this problem before in your sitting. Anxiety is another word for fear. Fear is a powerful and basic instinct, so it is a natural and sometimes helpful thing to have. However you must not let it take over your life. Hard as it is, we must all face our fears and leap beyond them; Only when we have, do we see that the fear was a lot less scary than perhaps we thought it was.
When we sit we are asking to see not only the best, but also the worst and most frightening parts of ourselves. Fear, Anger, Greed and Delusion. All of them are a part of us and all of them can be worked through. With time, patience, compassion, generosity and All Acceptance, you will find peace.
“It is easy to believe we are each waves and forget we are also the ocean.”
― Jon J. Muth
“Consider your own place in the universal oneness of which we are all a part, from which we all arise, and to which we all return.”
― David Fontana