What Causes Delayed Periods,brown Discharge And Fuller Breasts?

Hi there, I've recently missed a period but for the last two days been bleeding dark brown blood and had no cramps. I'm always on time like clock work. I've had nausea and sickness. my breasts have gone up a cup size and are sore and feel hard and look veiny. I'm always crying. I have acidy heartburn. my uterus is the size of a tennis ball and i'm bloated around my belly and my clothes feel tight. my back pain is so bad I'm in agony when I sleep. What can it be?