Sick after swimming

I was looking for some feedback or thoughts on my situation. This past weekend (Sunday), I competed in a Masters meet and swam 4 events (300 total race yards) over the course of 3.5 hours with a total of probably 2000 yards total swimming for the day. I was pretty beat after the meet as expected. Sunday night comes and I get a headache and nausea, no vomiting, fever, coughing, or runny nose, etc. Its now Tuesday and I am still have the nausea, better than I was Sunday night and Monday. Now here is what gets me, this same happened to me last year at the same meet, same pool, etc. I have done all sorts of training and racing in all types of conditions, from the pool to open water and triathlons since then with no adverse results like now. I have pushed myself in training and during the events themselves and I always hydrated and nourished. When it happened last year, I equated to pushing myself to hard but it hasn't happened since until Sunday. Also I learned that complaining to my pregnant wife about being nauseous for a day gets no sympathy :)