Can someone explain this to me in a way I might be able to understand?

I started getting some pain in my feet & ankles when I started playing netball a few weeks ago. In the past week, both my ankles have been swollen (not red) & I've had hours at night when I can't sleep because of the aching in both my ankles & knees.

After no improvement I went to the GP today & he's tested me for rheumatoid arthritis. I've been doing some googling & I'm a bit confused. I'm in alot of pain, & my ankles are both noticeably swollen, between my ankle & the bottom of my foot. The aching is right in the joints all day, but more unbearable at night. But its not hot & red. GP did say if it gets hot & red they need to know straight away...

I also read its an autoimmune disease? Is that similar to HIV & other diseases similar? As in the type that reduce immunity to infections?

Is this serious?? Should I worry?

I just have no real idea what to think right now