What Causes Inter Menstrual Vaginal Bleeding?

I am asking this question on behalf of my friend.
She had protected sex on 31st dec midnight and 1st jan and then on 3rd jan. On 5th jan, she got her periods after 26 days of previous periods which started on 11th dec 2014. These periods lasts till 10th jan 2015.But however she started bleeding on 15th jan afternoon and it was today morning that is 20th jan What could be the possible reasons for this bleeding? Please reply soon