I had that (but not as bad) at about the same time. I had a dinky wrist blood pressure thing to test at home and my BP was down. It was just after the baby turned so I think he pinched a blood vessel or something.

It was a bit scarey for a few days but eased up and I was fine the rest of the pregnancy. I'd go get checked out if it hasn't gone in a day or two or earlier its so bad you can't function properly. I couldn't drive or stand up long when I had the problem but was at home so it wasn't as bad an issue if I had been working or *needed* to drive anywhere.