Smoke from exhaust

As of yesterday I put on 250 easy miles and throughout the day my cousin behind me said when I get on the throttle it blows smoke from the exhaust pipe. Smells like un burnt fuel so I'm guessing I'm running rich but why all of a sudden now? 2400miles on the bike. I also noticed its been back firing more and more. I have a cobra slip on exhaust but have had that on since new. Any ideas anyone???