I did the running/walking part outside at 3:30 when I got home from work and then went swimming at an indoor pool a few hours later. After we finished swimming we went into the hot tub for a few minutes and when we got out I was about to pass out. I had to lay on the bleachers for a few minutes and apparently I was really pale. Lifeguards and other officials at the pool came up to me and gave me liquids and had to write a report and everything. It was embarrassing but it was my own fault.
The combination of the exercise plus the heat of the hot tub is what got you. Plus, maybe you haven't been working out much lately.
Also, try getting dome underwater. It's a little odd.