What you are experiencing is called "afterdrop". I've experienced it myself and it has been the subject of a lot of research. I wrote about it briefly toward the end of my recent post on The Iceman
, and there is a lot of discussion of this phenomenon in the book "Becoming the Iceman" by Wim Hof and Justin Rosales.
Here is what is happening: When you expose yourself to cold water, your peripheral veins constrict to shunt warm blood away from your extremities and to your core organs, to keep them warm. You will typically start to feel a bit warmer once this happens. But then, when you step out of the cold, the peripheral veins re-open and the cold blood from the extremities re-mixes with your core blood, so you experience a sudden cooling or chilling effect. This causes shivering, which can often last 30-60 minutes -- perhaps longer, as your case illustrates.
The good news is that the more you expose yourself to cold water -- colder temperatures, longer times, more contact with the water -- the more you adapt and the weaker the afterdrop effect. In my case, I get absolutely zero afterdrop from cold showers, but when I shifted to ice chilled baths a few weeks ago, I begain to get strong afterdrop. With time, my afterdrop effect has become much shorter and less severe, but I still go through a period of shivering. It is definitely unpleasant.
I've been experimenting and found several ways to really reduce the afterdrop effect:
1. Back off on the duration and intensity of the cold exposure. In your case, you could tolerate 4 minutes, but not 8 minutes. So go back to 4 minutes, or slightly warmer water, and gradually increase the time or decrease the temperature each time. If you find yourself shivering, back off and proceed more slowly. You WILL improve.
2. Exercise immediately after the cold exposure. You will still feed the cold rush of the afterdrop and you will shiver, but the intensity and duration will be much shorter.
3. Follow the cold shower with a lukewarm shower, and then slowly turn up the water temperature until it is very warm or hot. Do this gradually so you don't feel a "burn". You will find that you may need a fairly long warm shower to feel satisfied -- probably as long as the original cold shower. Wait until your hands and feet feel warm to the touch. This is evidence that your core and peripheral blood temperature are now equilibrated.
The fact that you are shivering for more than an hour after your cold shower is evidence that you are not cold adapted. But the good news is that you can adapt using some of the above suggestions, and you should experience a significant benefit in terms of becoming more habituated to cold weather.
See what works for you and let us know. I suspect that there are a lot of people who could benefit from your experience.