Losing weight can effect your hormones (say if you don't ovulate(if you're a girl) ovulating more often will make you crave sex more often) which can effect your libido. I've also had more than one doctor tell me that losing weight can lead tor a better body image can increase your own sexual feelings because you feel more confident. Also, you may just be feeling better and not loading your system down with sugar. Feeling better altogether over means the sex department to.