I missed a pill and two days later I was intensely cramping and light spotting, which has now become heavier spotting. Why and when will it stop?

Unpredictable. This is a direct result of missing the pill. It may stop shortly or it may persist for the remainder of this cycle. It is also best to use additional protection for the remainder of this cycle.
You missed pill. If you miss birth control pills then they do not work and it triggers irregular bleeding. This should resolve over time. More importantly if you are having trouble remembering pills think about a safer, more effective and easier to use option such as a Skyla or Mirena (levonorgestrel) IUD or Nexplanon.