When I was little, I could sleep through just about anything. Now, it doesn't take much to wake me up at all. What's with that?

I don't know but if you keep noticing further changes, mention it to your doctor.  Changes in sleep can be symptomatic of various things.  

You are nervous.

Same with me. Don't know why, but it does seem it doesn't take much to wake me up and I wasn't always that way. I woke up at 3:00 A.M. this morning when my roommate came home late and he was being really quiet too. At least I woke up when an earthquake hit a couple years ago...

Hormonal changes from puberty are largely responsible.  
There are other factors that could lead to it and it's worth paying attention to, at the same time though this is normal and to be expected as you finish maturing.

Asyou age all kinds of things change, believe me.

I am the opposite, I used to struggle to sleep... now I can literally sleep anywhere anytime...I nap all the time and still get a good night's sleep...