What Causes Abdominal Pain Along With Vaginal Bleeding Post Postinor Pills?

Hi pls am getting worried I had sex with my girlfriend on 5th of dec around 9 pm and she took postinor 2 around 12 pm the following day and the 2nd pill 6 hours later after 3days of taking the pill she had abdominal pains with bleeding for 3days she expected to see her period on the 2-5 of Jan Bhu it did not show waited till 15th of Jan Bhu it still did not show we had a blood pregnancy test of the 16th and it came out negative now it's the 23rd and still no period pls wat could b the cause?