My thought is that it would be best not to offer in the particular situation you gave. That is, it was night, and presumably there was no life-threatening danger to her from the rain.

Of course, there is nothing wrong with being generous, but there are always other factors to consider. If, for example, you want to hold doors open for people or offer to carry heavy things, that is fine, as long as you do that for everyone consistently and don't take offense if anyone refuses. Also, you may want to consider the context. Even if you are not a scary person, offering to help somone with a minor task if the area is dark and/or deserted can be perceived much differently than in a more typical context.

I would advise you to continue to make the effort to recognize when you may be conforming to undesireable cultural norms, as you have been doing here. That is the first step to taking action on this extremely pervasive issue.