Eppy, you are only seventeen! I don't think you have anything to worry about.
Your body isn't done developing yet, your penis will continue to grow for a few more years. I've never heard of penis enlargement pills, creams, etc working.
As for it being bigger two months ago. I don't think its shrunk, I think maybe you might have gained a little weight and have a fatty deposit in your mid area that gives the illusion that its "smaller". Or it hasn't changed at all but you've got yourself convinced that its different.
Try this, hold off masterbating for a couple of days. (or no sex if you are sexually active). After a couple of days without doing anything, when you masterbate see if you notice it being any different than when you most recently did something,
i know with my fiance', if we go for a few days without anything going on when he gets aroused he appears fuller and bigger than when we do it more regular. And that might be your problem.