What Causes Delayed Menstruation And Blood With Mucus Discharge?

Ive had irregular periods but there usually only a day or 3 days late, its a 28 day cycle and quite heavy, 2 weeks after I always get bloody mucous and my doc said its an ovulation sign, I had sex twice around that time no condoms and once a few days before im due my period, im now almost 4 days late and I noticed mucous with blood spots n it on the day I should have had my period which ive never experienced before, ive had sore breasts and cramps and did a pregnancy test although I was told its to early and I got a negative result, could I b pregnant or whats going on ?