What am I doing wrong

I'd second the snack immediately after the ride. Honey on warm toast and a cup of tea is my favourite.

Well done on trying to lose weight!

If it's happened a few times and it's consistently about 30 minutes after exercise and there are no other symptoms it's most likely to be low blood glucose that's causing the problem. If you're trying to lose weight, are you eating enough of the right carbohydrates to keep you going for a training session in addition to normal life? Making sure that you don't go without food for more than a few hours before a training ride and then having a small snack such as dried fruit or a banana when you return should be sufficent to stop this from happening and still allow you to lose weight. If you're already doing these things you certainly should see a dr but I doubt that there's anything seriously wrong.

I presume you're not on any medications, prescription or otherwise?

What do you mean by passed out? Collapsing? Unconscious? Falling asleep?

How old are you? Any medical conditions? Any other symptoms? Ever done high level exercise before? Similar problems?

Don't bother with NHS dire, go to see your doctor would be my advice.

...also you may be a bit low on sugar, my lady has low blood pressure and sometimes she gets faint after she runs, a sugary snack seems to help

I'm not a doctor but I'd suggest seeing one - or at least call nhs direct.

i'm not a doctor but it sounds like good old fashioned sunstroke to me. A light coloured long sleeved top and a wide brimmed sunhat would be my prescription, and keep drinking the water...