my T suggested i have free floating anxiety. basically sometimes it just happens for no reason. that may be happening to you too.
often times i have learned to identify other forms of anxiety caused by a trigger. then i can analyze the cause and face it head on or put it into a rational thought or action to reduce the anxiety.
exercise for some helps. i usually mop or vacumn to get my mind off it if only for a few. helps my mind "recharge" or disrupt the circuity. for me sitting very still helps too. helps calm me down. i try to clear my mind of any thoughts during the sitting. if a thought arises i acknowledge it then return to a still state.
hope these things may help you too.