so this app has always had my periods on track or sometimes I may even start a day early but anyways it said I was supposed to start today which I haven't

and I've been having tons of nasuea and I already have two beautiful boys (one with the guy I'm currently with) but I had them when I was younger (15 & 18) and I never kept track of my periods and I was also 3 months along when I found out with both of them so now I'm not sure what to do. My boyfriend and I don't use protection so there is a possibility but I don't know if I should be freaking(getting excited and anxious) or what. is there a chance or could my app just be wrong!? also when should I buy a test if I don't start