The symptoms you are experiencing are quite normal during pregnancy
and they are due to excessive metabolism
going on in the body due to pregnancy and the associated hormonal
changes. Electrolyte replacement, plenty of oral fluid intake, sufficient rest and some walking if possible along with the usual prenatal medication will suffice. Since you have already had three pregnancies, it is possible that your general condition is a little weaker, hence the weakness may be more. Have regular visits to your doctor and follow advice. Wish you a safe delivery.