My gums bleed every time I brush my teeth.

I didn't used to do this before, but lately my gums bleed every time I brush my teeth. I don't brush that hard, but it still happens. Is it because my gums are weak? Or is there a problem with my teeth? Upon closer inspection, it seems that the bleeding occurs in specific areas. I tried brushing very gently once, and there was no bleeding. So, should I just keep brushing gently? I'm not sure if I should continue with gentle brushing or visit the dentist for gum treatment.