She is looking for your empathy - a verbal expression of it, and what makes you think you must withhold this from her? You say she is a long-term friend, so presumably you share a certain level of confidence in each other and she is opening up to you for a reason.
As others have expressed, I am concerned that you make a judgement that her pain and case is mild. How is that relevant to measure out your compassion anyway? A child skins their knee. You still give them a kiss and a band-aid to feel better. It is your expression of empathy at the time of acute fear or pain that does the healing more than anything else.
Perhaps you require additional information about Rheumatoid Arthritis in order to provide context for what your friend faces. RA is a chronic progressive auto-immune disease that will never go away and has the potential to disable her at some point. Here is more info to help you understand:
Understanding Causes and Risks of Rheumatoid Arthritis (RA). And as heart says, she may be frightened by her illness or the idea of IV infusion and just needs to talk about it, to reassure herself or to someone who might be able to reassure her. I get that
you feel like you would be imposing on others to discuss such issues, but if you really were burdened by something, would you open to no one?
You wonder why she doesn't feel like an "attention whore" discussing personal issues - I get you are using this phrase to be a bit provocative, but this would make me inclined to believe one of two scenarios about you: 1.) You have never been seriously ill and actually needed to reach out to others or accept help or 2.) You have been ill and no one paid any particular attention to your needs at that time so you have concluded that you must go it alone. Perhaps her expression of illness makes you want to repel her somewhat as she represents the reality of illness and ultimately our own mortality. Or perhaps you are tired of people in general gravitating to you and abusing your receptivity to listen. I don't know - you tell me.
But I
can tell you that you are making too many assumptions, and even your assumptions are flawed in the level of response you could be providing to this person. I would like to see you amp up the empathy for her. Did you consider that you might be the only person that can bring her some peace of mind on this? You have the power here to help her feel a bit better, so why not?
Share with me why you feel annoyed to hear about her issues, and I would certainly love to dig deeper on it.

Fabulous Udog.