Resolution Issues

So, I started the task of embarking on a new and healthy diet yesterday and have had a major breakthrough.
I have realised - it is actually healthy food that is making me ill!
I spent 24 hours on a lower calorie diet, eating healthy food only - i felt so ill last night, I couldn't sleep and eventually nodded off, totally exhausted at 7am.
Today, I broke the diet and ate a few small chocolate biscuits and for dinner - chicken and chips, I still feel unwell - but no where near like I did last night and I actually had more energy!
Just to make sure it is not a fluke - I will try and do the healthy diet again tomorrow, but if I feel the same in the evening, I will certainly not be continuing with the healthy eating plan and will try and see a Doctor to see if they can tell me more.
It's very frustrating, I want to change my lifestyle but how can I when it is affecting my 'Quality of life'?
By the NHS standard, I am still 4st overweight - but what if I am not?
What if I am actually 'built' to be a certain weight (bigger than what is considered normal) and by attempting to change that will ruin me?
I don't feel unhealthy, fitness wise - I mean, I can't run miles and miles (Not everyone can) - but I can easily walk a mile now without stopping or feeling tired and puffed out (It used to take me 33 mins to walk a mile - I can do it in 11 mins, without stopping and at a faster pace!).
I walked a Mile today.
For now, the diet plans will be on hold somewhat - but I am still determined to do the exercise part of the plan to build up better fitness.
I have a rough plan of everything, but the best way of 'planning' is to take each day one at a time.