Bleeding two weeks after period ending

Totally new to here but just want some answers from people who may have experienced similar. Period started on 1st of August and ended on the 4th, sexually active since the 10th of august up until the 18th when I woke up and noticed brown discharge and red spotting. Wasn't coming to anything and was only when I wiped.  Had sex few nights later and the blood got heavier and was varying between light and heavy with a few stringy dark clots,  now it is pretty heavy but not as painful as my normal period apart from my lower back.  I have took a pregnancy test and is coming back negative I have never had two periods in the one cycle and just confused as to why this is happening. Hoping to get some answers from here x