Why Did I Have Spotting After The Period Was Over?

I had my period over a week before going on a trip to the mountains (10,000 ft above sea level). After being there for 4 days I found blood when going to the bathroom. It was more than a spot, but less than the first day of period. Could it be caused by the high altitude? dryness? It doesn't hurt, but I am panicking, can't sleep. Please help! I am 31 years old, I am overweight and have never had any problems like this. I am in a monogamous relationship for over a year now.