First of all, you need to confirm whether you're pregnant or not. You can do this by taking a pregnancy test. Even if you're miscarrying, it will still be positive - the hormones take a while to leave your system.
If you
are pregnant, heavy bleeding is a very bad sign. It usually indicates a miscarriage or a threatened miscarriage. You need to be seen by a doctor if you're pregnant and bleeding. An early miscarriage can't be prevented, but sometimes there will be parts of the pregnancy that remain behind and give you an infection if they're not dealt with.
The symptoms you described sound like PMS symptoms, actually. Why do you think this isn't your period?