Abnormal vaginal bleeding for 5 days

[Milliec]

I have been on the contraceptive pill for 2 years now which had made my periods much regular/lighter/shorter. I am 15 days into my pill and am bleeding which is making me become worried as I am only in my teens. I went to the GP and she was adamant the most likely factor was a sti however I have been checked recently and all came back negative so I know that's not the reason.

The blood is a brown colour and then yesterday became red, however now has gone back a darker colour.