What exactly could this dream mean?

You have a fear of spiders, that is what Arachnophobia is. Your subconscious is just reminding you of your fear and that you are trying to deal with it, that's why you ran to get your mother, but you are not getting the help you need, that is why she didn't come to help you get ride of the spider. Your fear is normal and there's nothing bad about it. Seek help for your fear and take it a step at a time.