Hello, thanks for using Health Care Magic.
There are a hand full of causes of tachycardia and you have already excluded some with the work ups done so far ( electrolytes or minerals, anemia, thyroid function).
Sinus tachycardia usually is in response to physiological changes through an increase secretion of adrenaline from the adrenal glands due to stress or fear or anxiety. People who have anxiety or stress have increase adrenaline secretion which could still be acting even when there is no stimulus for fear. However, apart from stress or anxiety, other pathological conditions can also account for sinus tachycardia such as phaeochromocytoma (which will present with palpitations, increase blood pressure and heat flushes) and other endocrine growths. 140/98mmhg doesn't fall within the normal range of blood pressure and hypertension is known to cause tachycardia.
It is also possible to have intrinsic causes (related to the heart ) of tachycardia such as abnormal conduction pathways, post myocardial ischemic lesions, ectopic foci etc.
In your case, what you need to do is to meet a cardiologist for a proper examination and further testings including ECG, adrenaline and retake of bp measurements at least in two different times.
Best regards and hope this help.

Dear patient, normal heart rate in an adult person ranges from 60 to 90 beats per minute and a heart rate more than 90/minute is called tachycardia where as a heart rate of less than 60/minute is called bradycardia. Normally these heart beats are due to the impulses generated by a center in the heart which is called sinus node or pace maker of the heart and if the pace maker of the heart generates impulses at a rate of more than 90/minute it is called sinud tachycardia and if it does it at arate of less than 60/minute it is called sinus bradycardia. There are various causes of both sinus tachycardia and sinus bradycardia. If the heart beat impulses are produced by the areas of heart other than its pacemaker or sinus node it is called arrythemia and if the rate of such heart beats is more than 90/minute it is called tachyarrythemias and if their rate is less than 60/minute it is called bradyarrythemias. Tthere are different types of tachyarrythemias depending up on the site in the heart from where the impulses are generated and the pattern in which they are generate like atrial tachycardia (unifocal or multifocal), atrial flutter,atrial fibrillation, paraoxysmal supraventricular tachycardia, ventrivular tachycardia. The only way to differentiate between sinus tachycardia and tachyarrythemias is by doing ECG or by seeing the ECG complexex in the cardiac monitor . Causes of sinus tachycardia includes exercise, fever, pain, anaemia, fear and anxiety,nervousness, excitement, dehydration due to fluid or blood loss, hypotention due to any cause, infections like pneumonia, drugs like bronchodilator cough syrups, excessive tea, coffee,smoking or alcohol intake, use of drugs of abuse like cocaine and amphatamine , alcohol or other drug with drawls, heart failure, hypoxia, hyperthyroidism ,epileptic seizures, hypoglycemia , etc. Tachyarrythemias are caused by some underlying heart disease like ischaemic heart diseases,valvular heart diseases, cardiomyopathies etc, lung diseases like chronic obstructive lund diseases, drugs, hyperthyroidism, electrolyte imbalances etc. In the presence of these conditions a normal sinus rhythem or sinus tachycardia can change into tachyarrythemias. In your case if the tachycardia is persistent ,get an ECG done to see whether it is sinus tachycardia or some type of tachyarrythemia like atrial fibrillation, if the tachycardia is intermittent ,get an HOLTER MONITOR done to see if it is some tachyarrythemia like paraoxysmal supraventricular tachycardia. Secondly a Chest Xray and echocardiography will be required to rule out any underlying heart or lung disease. your blood test are normal including thyroid function test. Check your blood sugar level at the time of tachycardia episode.If you don't take any drugs or medicines or have abstained from any drug that you were taking previously and also if you do not have epilepsy then your tachycardia and fainting episode is most likely due to panic attack with anxiety neurosis for which you may require a psychiatric consultation