(All Imperial units in my post, for a change...)
I went for a ride today. 100 minutes and 25 miles.
About half an hour before the ride, I drank about 16 fl.oz. of tap water and another 12 of fruit juice (pineapple, orange, banana mix, no added sugar).
The temperature outside was around 55°F and the sky quite cloudy, almost overcast at times.
I didn't sweat much at all (I never do...) and I didn't feel excessively tired.
After the ride, I ate two vanilla/choc ice creams - this kind:
About 15 minutes later, I drank a 16 fl.oz. soft drink bottle (no caffeinated kind).
1-2 hours later, I gradually got a bad headache and felt hot, dizzy and nauseous, and my pulse shot up to between 100 and 125, from around 80-85 half an hour after the ride. My resting pulse is 55-60 normally, and maybe 65-70 in the evening following a long-ish ride during the day.
I drank some more fluids 4-5 hours after the ride (2-3 hours after the headache started), and now I feel pretty good again. Took some paracetamol too to take the edge off the headache.
My pulse is down to 60-65 and I'm cool again.
So, was this a clear case of dehydration?
Or something else?