Having on and off heavy bleeding other than normal menstruation. Why?

Hi doctor,

I am 24 years old. I am not on any sort of birth control pills. I had a normal menstruation in the starting of this month and normal bleeding. After six to seven days, I started bleeding again. The bleeding was not very heavy, but lasted on and off for about six days. Now, after a gap of four days, I have got heavy bleeding again. It is not brown blood as I normally see during my period. It is dark red, much thinner and flows readily like a liquid. I feel pain in the lower abdomen whenever I get this bleeding. I also feel weak. There is no pain on urination or sex. I do not have a history of any infections. Also, I am not on any other regular medication. I am pretty sure that I am not pregnant. I have got a GP appointment in a week, but it is really worrying and painful sometimes. What could this be?