Hi there, and glad to have you here. The only person that can diagnose you for Lupus is a Rheumatologist. You have to ask your GP for a referral to see one. Before your appt, write down all your symptoms for the dr. to have. Let us know how it goes, Best wishes.
Welcome to the site and I think you've found a very good place to turn to...
there are many helpful, knowledgeable and sympathetic "ears" on this site!
It sounds as if your doctor is suspicious of an autoimmune disease cause for your symptoms and has run some initial tests (probably including the ANA=anti-nuclear antibody test). So you have a good GP since he/she has already thought to test you for this. With your symptoms, I think a visit to a rheumatologist would be a good idea whether or not these initial tests ordered by your GP come back adding weight to the suspicions or not.
I was a bit down too when my GP told me she suspected lupus (after my ANA test came back positive and I had been feeling quite ill for a few weeks and the red facial rash for months before that). Try not to worry too much... many have their lupus well controlled with medications so even if you are ultimately diagnosed with it it's really NOT the end of the world.