This means "something", and you should be on your toes. "Nothing" is usually used to describe the feeling a woman has of wanting to turn you inside out, upside down, and backwards. "Nothing" usually signifies an argument that will last "Five Minutes" and end with the word "Fine".
没什么意味着“有什么”，而且麻烦大了，所以你要保持警觉。女人说“没什么”的时候通常是在想怎么来来回回折腾你。“没什么”一般表示一场争论要持续“五分钟”，还会以“很好”作为结束。