Why has my period stopped after a day?

I’m 17 years old and me and my boyfriend have been sexually active for about three weeks now. We usually use a condom ever time but the last time we had sex we completely forgot to put it on so in the end he ended up pulling out before it came out.

Now I had just started my period that morning and everything seemed fine when I went back to bed that night. But the next morning I was in the bathroom and I noticed that my bleeding had stopped - completely.

I’m thinking that this is probably just because I’m majorly stressed out right now with school and friends but my boyfriend is sort of freaking out. He’s convinced that I’m pregnant because my period was there before we had sex then gone the next day. Now I’m doubting his theory but I know that I can’t be 100% sure since we didn’t use a condom.

If I were pregnant when do I take the test for it? I know people say when your next period is due but that’s roughly a month away from now since this one just started and I seem to have no clue as to when my periods actually start. (I used to know but I got sick once and it sort of threw my biological clock off).

If I were to wait a week would the test still work even though it’s not my ‘next expected period’? and does anyone think that I could be pregnant?