No, it is NOT rude for you to take time off for your medical issues. Can I emphasize that enough? Not rude! You need to take care of yourself!
Co-worker doesn't like it and is complaining? Well, too dang bad.
If she keeps complaining, I would pull her aside once, tell her in no uncertain terms how much unsupported work you had to do to cover for her and how you put your life on hold to cover for her... and after that I would report every instance of harassment from her to your supervisor and to HR. Honestly, pulling her aside and telling her to cut it out could be of benefit to her, as it could help her avoid disciplinary measures from HR.
When you do go back, I wouldn't say anything to co-worker. I would also never again bend over backwards to cover for her.
And with that out of the way, now a quick note on behalf of co-worker - sometimes people who have chronic conditions/pain have problems with empathy and have difficulty understanding that someone else's condition/pain is as important as their own. However, this does not excuse her rudeness and nastiness. And your boss and HR absolutely need to know how she is acting.
I hope that you feel better soon.