There are many possible reasons for this.
Most often the reason is hormonal imbalance.
Then there can be problems with the uterus such as fiboids, which bleed.
Liver problems, thyroid problems and kidney problems can upset the balance of hormones.
Endometriosis and infection can cause bleeding
You need an exam now to narrow this down, and to direct any needed lab testing.
Also make sure you have not gotten pregnant which can cause a light implantation bleeding, which would explain the more recent bleed.
OK, so you might have more questions or want to give me more information: Please use reply to expert if you have further questions. When you are ready, please click a positive rating [hopefully excellent—that’s how we are paid, per rating]. If you forgot something, come back. I am here http://www.justanswer.com/medical/expert-dr-thomas/