Hard getting a diagnosis - muscle pain, tiredness

For the last 2-3 years I have been suffering from the following symptoms - muscle pain (also tightness, cramps, heavy/tired legs), extreme fatigue, light headedness, difficulty in concentrating.
The muscle pain is mainly in my legs but sometimes I get it in my arms and cheeks in my face.
The symptoms come and go but are mainly worse during summer months. When the symptoms are bad I find it difficult to do "normal" activities that a 28 year old would be able to do, such as exercise or even going out for a night. I mostly go to work and come home and rest because I am so tired after the day. When I am good, I am still never 100% and it seems that if I do any strenuous exercise it brings the pain on.
I'm finding it difficult to get a diagnosis. I have been to a few different consultants, each one of them telling me they can't find anything wrong with me. I've also had numerous bloods taken, all showing up clear.
I'm at the stage where I'm running out of ideas on trying to figure out what it is, and I'm finding it very frustrating that I can't get to the bottom of it and continue living a normal life.

Is there anyone here that has experienced symptoms like these and is finding it difficult to get an answer? Or has anyone had these symptoms and managed to get diagnosed?

Any help/advice would be greatly appreciated.