Every pregnancy can be different. Have you highlighted to your obgyn?
Is your blood pressure fine? Or perhaps anaemic?
I had very low blood pressure during pregnancy and was anaemic in the last trimester. Therefore, I was lethargic throughout. Have to go slow when moving from lying down/sitting to standing position.
Eat regular small meals through the day. If hungry and blood sugar is low, it is easy to feel giddy too. Take care.