17 day period

Currently on day 17- GP have presctibed a tablet to collect monday to help it stop, but im hesitant as i want to let my body sort its self out as my GP said after taking the tablets for a week- im likely to have another week bleed before hopefully returning back to regular- meaning i would have been bleeding for over three weeks. My main question is... has this happened to anyone else and what was the reason behind it? GP not willing to investigate as its first time its happened