thanks for writing us..
your problem seems due to presence of some extrasystole that may be occassional or run of it.
sometimes theophyline also causes tachycardia.
if you are feeling this more when you are getting up from sitting or lying down position, indicates postural hypotension reflex tachycardia
You consult your physician/ cardiologist and get your B.P checked both in sitting and in standing position( immediately and 3 minutes after standing).
hope your problem will be solved