Maybe some form of trigger in your day or thought process that you'd been able to avoid previously. The urges are coming from your brain man, think of how long youve built up the neutral circuitry of this habit. It won't go away easily and it will fight you in anyway it can until it finally dies. Don't keep relapsing, you hit 15 days, appreciate the effort you put in to get into that point then get right back on track. Aim even higher this time, don't let it pull you back in.