No periods and pregnancy test negative.

Hey doctor I and my girlfriend 've had an unprotected sex on 25th September(Though not sex at all because no ejaculation occurred) which lasted just few seconds and of 5-6 strokes. But there was some precum envolved. Her menstrual circle is not fixed. Sometimes it is 35 days even sometimes it is 2 months. According to her previous period(i.e. 17th September), her due period should be on 21-22 October. But No periods had occurred. On 17th October she had spotting for the first time which lasted 3 days and was of black color. We had pregnancy test at home on 21st October which came out to be negative. But still no periods (today is 27 October). She is really tensed. WHAT TO DO NOW? Should we take pregnancy test again and what if test came out to be negative and no periods still? Should we take blood pregnancy test? Thankyou very much Doctor. Plz reply fast I will really be owed to you