why do i get a monsterous headache (the next morning) after an hour's workout at the gym?

Dont believe all this new age detox nonsense. If you are working out hard you are straining your self and puting pressure on your body and brain. This is probally causing your headaches. See your doctor though just incase its an underlying issue because this shouldn't happen all the time.

You are drinking far too much water during your training and nowhere near enough during the rest of the time. If your personal trainer is allowing you to drink a litre of water in an hour then get yourself a new trainer. And you are dehydrated.

have you recently changed your diet from junk, to clean foods and no sugar e.t.c? if so its insulin levels and blood sugar levels out of whack which causes fluctuation in how you metabolise calories and glucose is formed for energy. which i reccomend eating some honey throughout the day, 1 teaspoon 2 or 3 times a day untill you can fend it off completely.

another factor could be bad oxidisation through low liquid consumption which means higher water intake aswell as green tea extract as supplementation. you also need to eat good complex carbohydrate to sustain energy levels and not cause fatigue.

Dehydration is your best bet and make sure your not knocking your head unintentionally like sit ups and things, too much constant movement isn't good.

No, you need to increase your water intake. The headache stems from dehydration.

You're getting dehydrated when you work out. You need to drink more fluids while you are working out.

you sound as though yuo are dehydrating.. make sure you drink plenty of water. .build up your fluid intake..

dehydration will cause headaches....

sounds like dehydration

dehydration - you have to drink water during your workout and during the day also.

you need to drink more and take vitamin and mineral supplements as you are lacking from them as well