Hi msearle
It could be due to a couple of things.
Lack of Iron - Get your iron and ferrin levels checked by your Doc. Take an iron suppliment a liquid one is better but usually nore expensive.
Blood volume - Your Blood volume will increase by about 80% by the time you are 36 weeks. Rest and take things easy until your body adjusts to the increase in blood volume.
Hogh Blood Pressure - Get this checked as soon as you can.
There are also a number of othere things that could cause the feeligs you are getting. If it is happening everytime you you do simple tasks and continues to happen for a week or more then it would be a good idea to get checked out