First pregnancy!

Hi everyone! So I just found out today I'm pregnant (yay!) But I'm having an issue. My job just resently added maternity leave and you have to be signed up atleast a month before getting pregnant. My deadline was 8/1   When I took the test today it said I was 3+ weeks, but when I tested last week I had gotten a negative. Is there a chance I conceived after august 1? I haven't gotten a period since June, but I know I'm not THAT far along.  I thought we were being safe, but clearly not safe enough lol! Either way we will be happy. :)