Could i be gaining weight due to water retention?

so i just got back from vacation..and i gained about 4 pounds. =( i worked out 4 days out of the 8 & went on the ellptical for an hour..and the rest of the time i was pretty active & always walking. i did notice it was extremely hot climate...and i was always very thirsty..so i basically chugged 2 water bottles within minutes. i noticed that i only urinated maybe 3 times per day if that...and barely. could i just be retaining water weight? how could i be able to tell for sure?