Why are my push-ups and sit-ups are getting harder? Muscles getting sore?

I'm 14. I've recently started working out a little. I am pretty skinny.. and I did not want to stay like that, especially since I decided I want to join the military after high school.

-Before I started.. I could barely do 10. And 15 for gym class warm-up was really difficult.

-When I started.. I kept doing a couple sets of ten on the weekends. I quickly got better at them.

-I went to 20. They weren't THAT hard.

-Then I started doing 2 sets of 20 every weekday and 200 throughout the weekend(still in sets of 20.).

-The week after that I felt the need for a challenge.. and I did 10 sets of 20.. and ended up doing 200 in one whole day. The days that followed my muscles were REALLY sore and couldn't do my daily 40.. then I started again (yesterday) and today and did my 200 for my weekend.

*So that brings up the first question - why are my muscles getting sore after working out now?

When I'm doing my sets of 20(actually, I switched to doing 25 today.. but its a similar experience to doing 20). The first 15 seem okay-ish.. then after 15(however many i do after) they seem like hell.

*Second question, why does it feel like I am getting weaker/able to do less and less each day? Is that how it will be for the rest of the year?

I'd really appreciate a lot of answers..

thanks(: