Dizzyness etc etc.

As someone who has not only taken a lot of blood samples and who also has to attend the local phlebotomy clinic regularly to hand over my own blood I think I can reassure you that it is VERY common. It has never bothered me but for a lot of people it is a new and frightening experience and they expect to feel dizzy - often subconsciously. You sort of expect having the needle stuck in your arm to hurt - and it can do, not going to fib about that. A lot of people can cope with seeing someone else's blood but seeing their own flowing down the tube is a bit different. I once had an elderly gentleman in hospital who needed blood samples taken. I hadn't even put the needle on the syringe (long time ago this was ;-) ) and was just looking at his arm to find a vein and he starting screaming I was hurting him with the needle and he felt dizzy and sick. 

If you react like that they are very likely to suggest not donating blood - not because there is anything wrong but because they haven't got the manpower to pick up people who faint and need looking after. Simple logistics. 

Whatever is wrong - your reaction to having blood taken isn't unusual so  you needn't worry about that.

Not uncommon at all. It happens to a lot of people when they give blood. The best way to help prevent it in the future is to drink plenty of fluids and eat a high protein meal before and afterwards. I had to do this when I was getting blood work every 4-6 weeks for 10 months. The fluids and protein help your body new red blood cells faster.  You may of just had what is called a minor vasovagal attack. My husband get these whenever blood is drawn and sometimes if he gets pain all of a sudden that pitches a nerve. “A vasovagal attack is a disorder that causes a rapid drop in heart rate and blood pressure, resulting in decreased blood flow to the brain and fainting. Vasovagal attack is the most common cause of fainting.” – I got that from a medical site. The attacks arn't that serious in themselves but being injured in a fall when passing out is the biggest threat. It's good to keep in mind if this happens again when having blood drawn as any doctor or nurse needs to be prepared so they can help keep you from getting hurt should you pass out. Best of luck. If anything your bloodwork might show what that cause was.  

It's happened to me a few times now.  I thought I was ok with it but obviously not....Yeah your right about drinking plenty of fluids........

Is it common for people in their mid twenties to become unwell or loose the energy/activity that they used to have??