Hook effect?

A few days ago I got my bfp on frer and the line has gotten lighter and lighter each day since, tests now come out negative but I haven’t started bleeding and I’m not cramping, I still feel pregnant. I’m also 2 days late for af.

Can you experience the hook effect at 4 weeks? I’ve read that it’s more common when you are carrying twins and my boyfriend is a twin (plus they run in his family) so could I just be carrying twins and experiencing the hook effect?