If you are sexually active, it is probable that you may have got pregnant and the symptoms that you have been experiencing may be a miscarriage.
The other possibility is a hormone imbalance.
Keep a record of all your symptoms and how the spotting appeared and how long it lasted for.
Make an appointment to see your doctor to get your hormone levels checked out and any possibility of anaemia.
If you do start to experience a lot of pain with the bleeds get seen to urgently.