Hiking ... And sickness?

Been on a hiking trip all weekend. Three days, from Friday until today, from 8:30 am until around 4-5 pm each day. It's been relatively sunny all three days, so I've put on a lot of sunscreen, and been sucking down bottle after bottle of water. Today, I've had about 3 or 4 16 oz bottles from 8:30 am until 5 pm. We were outside all day. Ate lunch at 1, sandwich, 20 oz. pepsi and some chips. Got home tonight and feel absolutely awful. My head is pounding, and I feel sick to my stomach. Slightly dizzy, but could just be my imagination. Not sure why I feel bad tonight. I've felt fine every night except tonight. Extremely congested tonight, as well. Slightly sore jaw. Did I not drink enough water today? Or did I perhaps catch something from being out? Just seeing what may be going on. Ideas?