It is wonderful to identify the enemy and its associates that invade our lifestyle. I have traced this invasion-diligently for many years. It comes to me when I continuously focus on any mental activity eg. writing on the computer over a few hours.  My lips are tight, teeth are clenched and tongue is jammed behind the teeth; everything  is tense. My focus is so intense that I lose the rhythm in my breething. My eyes and mind are glued on to the activity. After 3-4 hours, I am overwhelmed by the anxiety syptoms you describe. I become irritative etc. These get followed by depression. I do not like anti-depressants-though I have taken plenty of them undermedical advice but the side effect on my libido deters me from using them. Instead, I use breathing exercises -inhale, exhale rhythmically. Often, the anxiety vanishes. The other approach I use is to be with somebody and have entertaining conversation  without  touching the subject  of anxiety.  Joy takes over- until the next session  of focussed  work/activity.  Medical causes over behavioural ones  have been  over-rated in the understanding  of anxiety.  Maintaining the breath rhythm is the master key  to avoid  the onset  of anxiety.