Aw, bummer, dude... I can so relate. I
hate having to explain to why I can't/won't eat certain things! I don't think I will EVER be good at conducting the whole "I'm trying to eat healthier" conversation; it always just makes me feel so awkward, and people have a way of making you feel like you're a total party pooper just because you don't want to participate in the junk-eating. I usually try to avoid explaining it, if I can--in fact, over the summer when I was just starting out, I would tell my friends that I didn't want to go out for ice cream/Taco Bell/whatever other junky thing they suggested because I was saving money for the vacation I took at the end of the summer! Hey, at least it worked and got them off my backs about it!
Is there any way you can decline the pizza without having to explicitly state your reasons? Even if it's just something as simple as saying that you had a very big breakfast and aren't very hungry (or you're planning on having a very big/special dinner and don't want to spoil your appetite)? Or just say that you have an upset stomach, or a sensitivity to the cheese on the pizza, so you'd rather not eat anything too heavy? Heck, you could even say that you just don't really like that brand of pizza because they use too much sauce/too little cheese/the crust is too thick/______. It seems like there should be plenty of ways to handle the situation without it having to be TOO awkward. Good luck, chicky!