Look up vasovagal response and orthostatic hypotension (You said your blood pressure "bottomed out", which would be the latter of the two.)
If you're having a panic attack and pass out, it could be due to breathing too rapidly, in conjunction with shallow breathing and you will pass out so your brain can "reset" your respirations, a.k.a. your breathing. If you're having a panic or anxiety attack and are prone to passing out for any reason, stay seated during the attack and practice mindful breathing to avoid this response as best as possible.
