Michelle,
I have been like this for a while now during this pregnancy... It is mostly when I am in shopping centres etc... I spoke to my midwife about it and she said it is to do with a few things... one is low blood pressure, another is low blood sugar, another is that shopping centres etc have only air conditioning and no fresh air, therefore, they are stuffy environments.
I was advised to always have a drink with me, and to rest when I feel light headed... and if it comes down to it, avoid shopping centres altogether. I have found the best thing is to have 2 drinks with me... one of water, the other of juice... the juice is great for low blood sugar.
I hope you start feeling better soon
Lisa