what gives? 😔

so yesterday I got 3 vf lines on 2 FR and one Wal-Mart cheapy. today I tested with a Wal-Mart cheapy and not even a faint line. it wasn't a FMU or a long hold. idk when I ovulated or anything because I haven't had a period sense June (I had a baby in April) we did the deed August 26th so if I am pregnant I'm only 3-4 weeks. am I still testing to soon? I know hcg doubles every 2 days but shouldn't there have been a line at least a faint one? I'm driving myself crazy