Please help

PLEASE HELP 🚨 im freaking out … someone please give me some insight or advice so I got a very very very faint positive on may 15th (the my period was due) today I’m supposed to be 5 weeks & 1 day according to my last period (April 19th)

I have been having slight cramping so I went to the ER & my HCG levels are 661 which they told me was pretty low they gave me an ultrasound & told me they didn’t see any sign of pregnancy AT ALL so now I’m worried about an ectopic pregnancy I have to go back Monday for them to do another ultrasound & to test my HCG which even if levels are rising the pregnancy could still be in the wrong place am I wrong? has anyone ever been to early for them to see anything not even a sac & still had a successful pregnancy?