What Causes Vaginal Spotting, Tiredness And Back Pain Post Menstruation?

My period are irregular. I would usually have 3 regular periods back to back, then 2-4 weeks late for the 4th period( this is year round) . I had unprotected sex with my husband 1 day before my period ended, which is common for us. My period ended 5 days ago. I woke up today spotting, extremely tired, back pains and cramping. This have never, I mean NEVER happened before. What can cause this? Should I worry about something? Is this a pattern of infertility?