Might be just the G. You have to brace yourself otherwise the blood drains from your brain. The feeling is similar to when you have been lying down and get up quickly.
It very quickly stops when you have done a few more aeros and if you are doing them yourself as you brace yourself almost automatically as you pull the stick. Sitting there while the instructor does the manouver is often worse than doing it yourself.
Do make sure that you have has something to eat within the previous two hours and have has a drink of water. Low blood sugar and dehydration make it much worse.
ZA
Glad you enjoyed it... funny how aircraft don't plummet when you do these strange things with them...straight and level will never seem the same again.