I think the 10 hours of work every day and looking after a small child when you are pregnant are too much for you. That is one reason you are feeling dizzy. The summer heat is an additionally factor, if you say your BP is OK. Hope you do not have a tendency for diabetes (because of family history). I also suppose your doctor has made sure your baby is growing satisfactorily.
This is the time that the baby grows maximally (in terms of weight). A small for date baby can face lots of problems before, during and after birth.
In view of this, and your work load, I feel you should go on maternity leave right away. You are paying a big premium to have a healthy baby - end result should be good for both the baby and you. Adequate rest and sleep, a good diet will help you and your baby. Keep away from the hot exteriors. If your house gets hot, install an air conditioner (if possible). Drink lots of fluids - tender coconut water, barley lime water (available as a drink in most drug stores), glucose-d with a dash of lime and salt etc.