I should think its because your big and requires more oxygen that's why. Atleast that's what my Dr said
I have asthma I think that the growing baby is making it harder to breathe. I would think that the reduction of oxygen to your head can give you a weird feeling such as dizziness or lightness.
I would call a Dr on call about the weird feeling in your head. Then you can ask about the Braxton Hicks too
I have been getting shortness of breath during my pregnancy for a few weeks. I have 8 days left.