Hi, I had gone through your question and understand your concerns. There are two possible conditions that may cause your symptoms:
1. Altitude sickness
2. Postural hypotension
Postural hypotension is the possible cause only if you were doing fast maneuvers with glider. Most possible cause is altitude sickness that occurs mainly in highs > 6000 ft. but can occur in lower highs.
I think in your case your symptoms are result of a combination of two conditions mentioned above. If you do gliding next time, I recomend you a good rehidration with plenty of fluids in the day and hours before you start gliding. Hope this answers your question. If you have additional questions or follow up questions then please do not hesitate in writing to us. I will be happy to answer your questions.