Nausea, feeling hot, dizzy and almost passing out. What can it be?

well, its not me. My girlfriend felt this way today, she was at church when she got up and realized that she felt like throwing up, soon after she sat back down and felt really dizzy. She got back up since church was over and her friend had to help her sit down and she couldnt even use her phone due to a blacked out vision. This same thing happened to her about 11 months ago. She frequently gets bad headaches usually in the forehead area and gets really dizzy.
At first i thought it was side effects of the Depo-provera shot she takes, a form of bc. And then i thought she might be pregnant. but, it can also be bad migraines. I'd like to know or get an idea of what it might be so if anyone can give me an answer or has anymore questions about this i'd be glad to answer them. i just need to know whats wrong with her cause its really stressing me out. thanks guys.