Here's the background. It's going to be long, so thanks for reading!

My coworker was off for 8 weeks due to "can't reference it cause she could look it up and find this and i'd be in trouble" pain in her hands and legs. It's a "flaring" pain. You can be in pain from it all the time, but the flare ups are 100 times more painful. And hers seems to be linked to her emotions. Her boyfriend has a terminal disease, and while he's still mobile now, he's not going to be cured. So whenever he has a bad day, she gets upset, and her pain flares, and she needs to call in sick. But when it's not flaring, she can work.
I have absolutely NO issue with this. I am sorry that she is sick herself, and i'm incredibly sorry that her boyfriend is sick. I've always been happy to cover her shifts, and help out whenever she needs it. Except for a few instances, I've been the one to cover for her. And I was the only one who covered for her the 8 weeks she was off, as our only other worker who could do the work was off themselves due to illness. And there was NOONE else in our place of work who could do "our" job. The only way I was able to get ANY time off was to pull in other people from other "offices" so I could have rests. So while I did have "time off" it was very regimented, and I had to cancel my OWN appointments, dr visits, etc, to make sure the work was done each day. Otherwise, I literally would have been working for 8 weeks straight, 7 days a week.
When she came back from being off, I was eager to see her, because I genuinely missed her. I was tired, yes, but I was happy to have her back, and be working with her again. I have never given her ANY indication that I was negatively affected by her being off.
End Background
During that time, I discovered that I had an ovarian cyst. It was hurting me, but I soldiered through. (and it was growing, and becoming more painful).
Our work is very physical, and it was getting more and more painful for me to work. To the point that I was on very strong pain meds just to get through the day. And it was affecting my work. Either being off the pain meds, and working through a pain haze, or being on the pain meds, and working through THAT haze. I was becoming really incapable to do my work to any positive degree. The product I was greating was poor, and not usable. It's obvious that I cannot work through the pain, or the meds.
After discussing it with my boss, she agreed that it would be best for me to go off work (onto a work supported disability program) until I was treated for the cyst. I've still been in pain even not working, and I'm incapable of bending over, picking up any object of weight, or sometimes even dressing myself properly) And it's also come to light that there may be something else in the cyst, and the word cancer has been used. But that hasn't been comfirmed yet. And if there's ever a day when i'm not in pain, i'm battling extreme nausea.
I have told my coworkers (all of them) this. I believe in being open with my life, and since my being off does affect them, they deserve to know. And my boss is keeping everyone informed with my progress, but I cannot control what she says / doesn't say. And they've all been supporting and comforting.
Except this one woman. Who herself was off for 8 weeks, with her own illness.
She even has the other person at her disposal to take over for her. I did not. So, while they have had to pull in the same worker from another "office" to help out, this woman still has an "in office" helper who is trained in our job, and can work for her if she needs another day off.
This woman is now rather mad at me, for my being off with this cyst. She has told me that "every woman gets them, they just go away on their own, you don't need to have surgery"
But even so, I'm still in pain. It's growing, not going away. And given our work, if it was going to break, it would have. And I've been to a gyno, who, after examining me, reviewing my records and such, is talking surgery.
So, (finally) my questions are these: Is it rude of me to be off, even though it's not convenient for another coworker? It does affect her life at work. And she never knows when she is going to have a flare.
Would it be rude of me to say to her "it's alright for you to be off sick, but not me?" I feel rather upset with this whole situation. I'm not well, and she's mad at me because she can't be off whenever she wants / needs to be, in her eyes. Almost that my illness isn't as important as hers.
Finally, does anyone have any ideas as to what I can say to her when I do go back? I don't want to start a scene, but I deserve to defend myself if she attacks me. And she's the only one there that isn't ok with my being off.
If anyone has any questions, or needs clarification, I will, but this became a novel already! Thanks for any help!