I found a good explanation here
@kiwiian,
Postural Tachycardia Syndrome - Raising Awareness of PoTS
Postural Tachycardia Syndrome (PoTS) can be a life altering and debilitating chronic health condition. Simply standing up can be a challenge for people with PoTS as their body is unable to adjust to gravity. PoTS is characterised by orthostatic intolerance (the development of symptoms when upright that are relieved by lying down). Symptoms include headaches, fatigue, palpitations, sweating, nausea, fainting and dizziness and are associated with an increase in heart rate from the lying to upright position of greater than 30 beats per minute, or a heart rate of greater than 120 beats per minute within 10 minutes of standing.
http://www.potsuk.org
It is worth reading some of the headings, especially the G.P. Guide as this gives some idea of treatments and therapies that may be of use to you..
http://www.potsuk.org/gp_guide