Health and workout questions?!

The past couple of days I have been VERY sore. I've been working out just about every day for the past two months, but the thing is I've never been this sore from it. I haven't made it more difficult, either. I actually didn't go for one day and I was a little sore, but then the next day after doing nothing it was way worse, and now today it's the worst it's ever been. Why does it keep getting worse even though I'm laying off the gym?

Also, when you're really sore doesn't it mean you're gaining muscle mass?

And lastly, I've already lost about 20 pounds and I'm at a stand still. I only want to lose 10-15 more, but it's gotten really hard. I lost the first 20 very fast; it surprised me. But now I've actually gone a few pounds in the worng direction, but I kind of think it's been from my few day of inactivity. Any tips on how to get these last few pounds off?
What I normally do when I'm not too sore is go to the Y and do cardio intervals on the treadmill for a half hour, then I do strength for a half hour, and walk or bike for about fifteen minutes. Also, I'm in highschool so I do a lot of sitting during the day, and aside from the Y the only time I really do anything is in gym class, and then I don't usually work up much of a sweat. I eat breakfast everyday, eat a lighter lunch, and usually have a small dinner after I work out. Everything I eat is also fairly healthy.
Do you have any suggestions? Any fat burning tips? Should I switch machines or try new foods? I just wish I knew what exactly to do differently to get where I want to be.
Also, I'm 5'7", 15, and 147 pounds. I just have a lot of fat I need to get rid of before I can truly consider myself healthy.