As most people have mentioned, the person who had contacted you may have not had cancer but rather it was just spam. I don't think it was legit because if she wanted money she could go on a GoFundMe or some funding website. It was in the spam folder, another reason I don't think it was legit.

Don't worry about being too nice. Yes, it is said that having of a good thing is bad but sometimes it can be good. Just be aware of the malicious people, and see if its worth helping or not. Other than that, continue being nice! It's something we all need from a person!