In your case, it sounds like you're not pregnant, especially if the bleeding you're experiencing has gone on for more than two days and if it has a flow instead of just spotting.
The pains in your lower stomach and back and the excessive tiredness aren't uncommon signs of your period approaching.
If you continue to have heavy periods or lots of pain and exhaustion leading up to your period that you feel is not normal for you, talk to your doctor. It could be a sign of anemia which is pretty easy to treat (I take iron-rich vitamins every day to combat mine!)
If you're still unsure of whether or not you're pregnant, it doesn't hurt to take a pregnancy test. You should wait to take one though until the bleeding has stopped.