heart rate and bp

Hi, i am 22 weeks pregnant, 25 years old. and in relatively good health. Over the past 6 weeks i have had spells of my heart rate getting up to around 145 (very dizzy and flush) and my blood pressure 90/62 at the same time. it usually last for about 30min-1hour. and then i feel fine again. It always makes me have a headache and makes me feel just awful.. what could be causing this?? could it be the pregnancy? It always seems to happen at work- after standing for about an hour.