Sore from pushups and sit ups advice/help?

This is frightening me. Monday the 6th when I got home from work I did 20 pushups and 25 sit ups. I planned on doing these every night. As I need to make sure I can do the 20 and 25 in under a minute for a physical agility test I need to take. The last time I went to the gym or did any excersise like pushups or sit ups was a few months ago. My problem and concern is i did that simple set Monday and now where my diaphram/stomach and sternum is, is incredibly sore like I spent an hour working my abs at the gym. I dont understand why doing it monday night has caused all the soreness but prior to that night Ive done it plenty of times before and never became sore. Is this normal at all? If you need anymore information just ask Im kinda frightened and dont think this is normal. Im a very healthy male 5'10 140lbs I walk a good 2-3miles a day at my job 5 days a week, I do smoke ciggs but will be quitting soon so I dunno whats wrong with me.