Feeling dizzy and faint the day after my first 20 Mile run - Training VLM

Hi, I am training for the VLM and felt fantastic after completing my furthest run yesterday of 20 miles. Apart from feeling sore last night all was ok. Then thismorning when I got out of bed I felt dizzy and faint. I though I would be ok after having breakfast but had this feeling all day and only now just starting to feel ok. I have been carb loading correctly, hydrating, Electrolyte drinks and gels whilst running. Lots of good recovery food and rest. Why of why do I feel like this. Is this normal? I've been totally fine in training up till now. Any ideas advice would be really appreciated.