Getting panic for pregnancy can happen?

We had protected sex on the 28th but due to the low confidence of both of us we thought that maybe the ejaculation could be some how done in inside, and we got panic so without wasting time, in the morning took an 'i-pil'.And It's been almost 13 days now, but suddenly in last 3 days she felt that may be her periods started so she used a pad like everytime at night she went to change her pad but she saw that its not like periods blood,its completely different just some spotting of blood in pads in brownish colour.And she also felt some backache.So we both got confused and feel panic again that how its possible because firstly we had  protected sex and then took an 'i-pill'.and  it started to hurt in the back like periods firstly.And with in 3 days, that spotting of blood get slowly slowly vanished.So we think that check with a pregnency kit and the report came negetive. Now we don't understand whether it's the side effects of Ipil or the symptoms of pregnancy. And the situation is after