Haven't been feeling well for the past few days and I can't figure out what's up.

BACKGROUND
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1) I live in SoCal, and we are in the midst of a nasty heat wave.
2) I have been diagnosed with anxiety disorder, and when I had my first panic attack I thought I was having a heart attack. As a consequence I now always associate my symptoms with heart problems, which typically doesn't help the situation.
3) I have a decent bout of hypochondria.

SPECIFICS
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For the past few days, I've been tired and feeling weak. Over the past few days I have also been waking with a minor scratchy throat, and a slight cough. As a result of the heat, I haven't been sleeping very well. While tossing during the middle of the night I frequently feel that my heart is beating quite hard, but I can't be certain that it actually is (as opposed to me just paying it an inordinate amount of attention). I've been feeling pretty hungry, and today -- which was the hottest day so far -- I was feeling *extremely* hungry before eating meals. Also have been trying to snack. Also, I have been feeling like I am 'spacing out' -- that is, slightly lightheaded, finding it hard to pay attention to my work, etc -- pretty frequently this week. I haven't had a fever; temp was a little high today (~99.5 deg F)

So, what's the deal? Is it the heat? Is it a cold or some sort of low-grade bug? Some sort of vitamin deficiency? I was at the doctor not too long ago (a month or two) and had a full checkup about 6 months ago. No chronic conditions -- diabetes, heart diseases, etc. I drink about 2-3 litres of water a day, but am sometimes concerned that I don't eat enough.