Welcome to HCM,
You are pregnant please do not worry as this is physiological state,and mild tachycardia
is normal, but if resting rate is so fast please check with your Gynaecologist and Blood pressure and Haemoglobin
level should be checked, if they are normal then other causes like Thyroid function and other test may detect the cause,even simple fever or dehydration
or simple anxiety may be responsible for fast heart rate.
Getting a EKG and 2 D Echocardiography
are p[possible means of detecting cause if there is any requirement.
Please take care of diet and nutritional supplements as advised by your Doctor like Iron, Calcium and Folic acid
Please continue to do physical exercise along with deep breathing exercises.
Wish you a safe motherhood.
Please Take care