Eowyn,

Have you been staying properly hydrated?

I was hypotensive for about 2 weeks 8 years ago due to inflammation in my neck. When the inflammation finally resolved, everything returned to normal. The tachycardia may be in response to the low BP.

Have you ever taken niacin? It can open up your arteries, especially your peripheral arteries/arterioles and capillaries.

Wiki says: Postural orthostatic tachycardia syndrome (POTS, also postural tachycardia can be caused by..... (SSRIs) such as fluoxetine (Prozac),