Hi - my theory (completely unsubstantiated and therefore quite possibly wrong) is not so much about the air pressure, but how muggy and humid it was. I have low blood pressure, and spent most of my teens keeling over in school assemblies and the like, and felt pretty wobbly yesterday lunchtime. But having fainted so many times I can now tell when it's coming at take preventative action (which involves simply mving around, making sure my circulation is going well, breathing deeply, and sticking my head between my knees as a last resort).