Miss I think you need to think of this as a marathon and not a sprint. You can't really just decide to be good friends so early on and then make that happen, good friendships happen organically and over time, and rushing in with over-intimacy, invitations, and so on can be too much for some people. Also, sometimes it just doesn't work out, you find you don't have so much in common, or you don't want to dwell on that part of your life, sounds like that might have happened with this lady, she gave her advice, but doesn't want to become a one-to-one confidant at this stage in her own recovery.
However, this is a really tiny blip, you for sure need to get out there, go to some different mum and baby groups, zumba classes, back to work part-time, whatever you want to do and let friendships develop along the way.
Also, sometimes groups can be quite bitchy, I found my NCT group weren't very nice as a whole, they were individually quite nice people, but sadly one or two were really quite bitchy and I found they were raising their eyebrows or talking about me and another girl behind our backs (not very behind, I could see it!). It was my lovely husband who said why are you going out with these people? I realised I was just a bit desperate to make friends and I'd rather clung on when in fact, they weren't that nice.
This may not be the case here, it may be that they are lovely but with this lady it just didn't work out. I would let her know that you are getting treatment feel much better thank her for her help and now move on. If one of the other girls seems friendly, go for a coffee with them but take your time and decide who you would like to be friends with rather than just rushing in. And, you will have opportunities to make friends for years- school gate, nursery, if you return to work, so don't panic, it will happen.