Skipping birth control pills or taking at staggered times can cause enough fluctuation to get you pregnant. BC pills work by controlling your hormone level. So when you don't take them regularly or at random times, the level in your body isn't level. This is why if you miss a day, most brands will tell you to double up on the pill you missed.
And even if you started your pills back, it isn't magic. It takes a bit to build up in your system. So if you take a pill today for the first time and then have sex tonight...you aren't magically protected.
An early response test can be used after a week BUT I would retest again at three weeks even if you get a negative. The test will only read positive once you've reached a certain hormone level.
May I suggest something like the patch where you only have to change it occasionally. It's ugly but at least there is less worry of you 'skipping' a pill. I also suggest using a spermicide with your BC pills as an added bufter. The younger you are, usually the more fertile.
And if you are 'flexible' with your pills, then keep condoms handy for those 'must-have' moments when you aren't fully protected. A whoa...need a condom might break the mood but so does an infant.