SOS havent had my last two periods!

I havent been on the pill for a good four months because it was messing up my body and the regulation of my period. My period is 31 days late (I missed my last period and Im currently supposed to be on my period, but Im not). I dont know if it could be from stopping birth control or if im pregnant. I have an appointment april 3rd to get Nexplanon injected, however you have to take a pregnancy test first. If i wait until the doctors to take the test, my mom will be there and will find out at the same time as me if im pregnant. So, id like to buy a pregnancy test before then and find out on my own.