Pregnancy or Period symptoms?

For about at least a week now I have had a sore back (also get twinges in my back), my are really heavy and sometimes hurt, I have been feeling really tired and my tummy feels constantly bloated. I have missed pills on the past and during this time and have had without a condom. I recently bled but it was just a browny colour and it lasted for about 4 days, what could this be?