Have you gained any weight recently? If you have, that could be what it is. I'm not saying that being fat makes your penis small, what I'm saying is that when you gain weight, you gain it all over your body, even if it looks like you just have a beer gut. So the pelvic region will have fat on it too, making it "puff out" more (I hope I'm explaining this clearly enough) so in turn, it makes your penis look smaller.
If you haven't gained any weight in the past few years, I have no clue why it would do that.
I hope this helps!