hi Lets first understand how blood pressure is maintained by our body ,this will help you to understand your symptoms better. Our kidney on top of them have to small glands called ADRENAL , these glands are responsible for secretion of hormones named ALDOSTERONE ,CORTISOL etc. Aldosterone along with Cortisol affects the body s ability to regulate blood pressure.It plays a pivotal role in maintaining blood pressure constant through out the day, It sends signal to organs, like the kidney and colon, that can increase the amount of sodium(sodium retains water and can increase blood pressure if there is a drop in blood pressure) the body sends into the bloodstream or the amount of potassium released in the urine.Aldosterone is closely linked to two other hormones: renin and angiotensin, which create the renin-angiotensin-aldosterone system. This system is activated when the body experiences a decrease in blood flow to the kidneys, such as after a drop in blood pressure,and maintains the blood pressure to normal again This was a brief explaination for how the system works a problem in functioning of adrenal can cause labile blood pressure and severe fluctuiation of blood pressure .so most likely your problem is either in adrenal glands or pituitarry gland which stimukate adrenals AS per the question asked by you antihistaminic cause mild hypotension and causes the blood pressure to decresase mast cell disorder has no proven relationship with blood pressure in documents,how ever antihistaminic in general mildly decrease the blood pressure my suggestion is go for a complete adrenal and pituitary check up for labile b.p under a endo crinologist