Abort! Abort!

I was planning on doing my overhead presses and squats today, along with whatever else came to mind after finishing those. The press went fine (3 x 5 x 75lb), and then I tried doing some inverted rows (not ready for those), and then I moved on to my squats. I did one set of 5, and as I racked the bar I started feeling a little dizzy. I immediately sat down on a bench, but the dizziness just got worse. It took a couple minutes for it to subside enough to where I felt safe standing again. I decided the prudent thing was to call it a day. But now I'm left to try to figure out what happened and I really don't know. Perhaps I wasn't breathing properly during the lifts (holding my breath)?? I only had a small serving of yogurt (full fat Greek style) to eat this morning, but I did the same workout last week on a 16 hour fast and felt great. It's now about an hour since it happened and I still feel a little fuzzy-headed. ??????????