I Recently Went On A High Altitude Trek. Due To

I recently went on a high altitude trek. Due to less oxygen I used to breath heavily and sometimes was breathless also. The thing I noticed was that I had started getting a pounding headache on one side. It was a throbbing and I could feel one nerve in my head which was causing the pain. This would normally occur when I would be climbing uphill and needed more air.. Can you please tell me as to why was this happening and how can i prevent it in future..