YEY! I did it and I feel so much better, thank you so much for your comments.
I said, "I think it's about time I explained what is actually wrong with me because I'm sure it makes you feel awkward not really knowing (cos they knew something was wrong but didn't really understand) and I feel it would be best for both of us if I allowed you to understand...
"I have Joint Hypermobility Syndrome, it means my bones aren't stuck together properly and my joints aren't supported properly which effectively makes my bones wobble around (maybe not the best way to put it, but thats how it came out) it causes my body to be in lots of pain"
"I manage to cope with the pain most of the time, but sometimes I find it very difficult, it is the times I find it difficult when I may miss work"
I then explained that when I am finding it difficult to cope with the pain I also find it difficult to discuss the way I am feeling, therefore when they ask how i am feeling i may give short answers, this is not because I don't appreciate you asking, it is because I am scared I will cry.
It was that easy. And I think she actually got it.
Now at least I know someone else knows, and when she asks how I am/I phone in sick I can explain to someone who knows.
I know not everyone will be this easy to explain it to, but it has certainly helped my confidence to tell others.
Thanks again for your comments guys...