I would therefore think that breath awareness is sort of expanding that anxiety into the area of feeling short of breath, as needing to pause significantly to breath when talking, and that general mindfulness is expanding the reaction into acute awareness of feeling a tight/clenched stomach.
it really sounds to me like these things were already occurring, but having a more acute awareness of them is having a snowball effect... anxiety produced by the thought of having anxiety and a more acute awareness of the physical symptoms.
this seems to be extremely common in people who have panic attacks... they have a little anxiety, and focus in on the heart beating and short breath, which gives them more anxiety and explodes into a panic attack.
the shortness of breath and clenching you are feeling, imo, is the result of an unmindful bodily response to the anxiety filled mindstate... this whole interaction between your mind and body is going on without any awareness or input from you. if you can learn to catch the mindstate with sharp mindfulness, and stop the physiological response before it gets out of control i think you will likely be able to stop this problem quickly as you haven't (said you've) been prone towards these attacks. the mind and body are intertwined and both can influence one another, the same way your anxious mindstate is having an effect on your body, conscious control of your body can influence your anxious mindstate. i try to control my breathing when i'm speaking in public, or having any sort of heated conversation and find it very effective.
I at times feel conflicted about whether I should "control" the breath or just "let it be" but the anapanasati sutta does refer to 'calming the body' if I recall correctly, as one of the contemplations.
some of the instructions in the Anapanasati sutta are directed at passive observation of the breath, primarily the 1st and 2nd steps of the 1st tetrad prefixed by 'he discerns'...which implies an element of passive observation. but the 3rd and 4th steps of the 1st tetrad are prefixed by 'he trains' which clearly implies an element of intention. and by intention i mean gentle nudging in a direction, as opposed to forceful pushing. my understanding is that tradition says the Buddha gave these tetrads in response to a monk who was simple passively observing the breath and thoughts as they arose.
i view this Sutta as basic instructions combined with a 'road map' to how Right Concentration begins to unfold. it's been my experience that concentration unfolds naturally exactly explained in the 1st tetrad. the breath starts long, the breath gets short, the breath expands through the body (or becomes a seperate, disembodied body depending on how you direct it), the breath calms down, rapture arises. i dont think all 16 steps should be worked through at once, nor do i think they necessarily have to be linear... i actually think, and this is my opinion, that one should stay with the 1st tetrad until rapture and ease arise naturally.
"It's easy for us to connect with what's wrong with us... and not so easy to feel into, or to allow us, to connect with what's right and what's good in us."