I have low blood pressure as well. My normal is 110 over 75 or something like that but with pregnancy it dropped to 80s over 40s/50s!!! One MW actually jokingly asked me if "I was feeling ok" MW seemed to have no clue but doctor wasn't worried.
as for why your dizzy from standing to sitting etc. its postural hypotension. just do it slowly so you dont faint!
and for why your headaches go away, you're probably slightly hypoglycemic, your blood sugar was too low before you ate. you need to eat more regularly and maintain your blood sugar at a normal level.