Oo-er..... there is a duty of care on your employer to make sure you are OK if you are off sick so it could be a sort of pastoral call - but of course there is always the possibility that she is being just as affected by your manager's attitude as you are. I think you need to be careful not to be drawn into any sort of mutual support arrangement, particularly while you're not feeling very robust yourself.
Why don't you email her and thank her for the voicemail and you appreciate the call, but you are feeling rather fragile at the moment and you'd rather leave it a couple of days before you speak to anyone from work? After all, you do want to concentrate on getting yourself better, rather than hearing about her woes - people don't always realise how unhelpful it is to say "I know how you feel, I'm there too" and launching off into their own problems - which don't help YOU.
I think you should respond (you are a dutiful employee after all, and need to cooperate!) If she's genuine and you have always got on well with her, there may be no ulterior motive, she may just be asking after your welfare - so if you do phone her, you can keep it short and say you're not up to a long conversation at the moment.
I hope this helps...