Should I be worried??

During my 1st week of this months pill pack on about the 3rd day I forgot to take my pill until 6hrs later that day. My pill packet said that if I missed a pill to take it as soon as I remember or just double up the next day so I took it when as soon as I realized I didn't. My pill packet also says that if that happens (I for get to take a pill but take it asap or doubling up the next day) that I would still be protected for sex and that I wouldn't need a back up method. I did have sex 4 days after that happend and now this week is supposed to be my period week and I've got an upset stomach, could this be a sign of pregnancy or am I just worrying/freaking out? Has this happend to anyone, what do you guys think??