PLEASE HELP!!

My last period started on September 5. I had unprotected sex the 20th, 21st, 22nd. Cycle days 16 through 18. I was supposed to start my period either the 4th or the 6th (today) and I still haven’t. I always have 31 day cycles, sometimes less, but never more. I’ve never had a cycle over 31 days. I’ve never been late. Even when I’ve been the most stressed out, I was never late. I’ve started early before but never late. I had a beta hcg blood test done on Wednesday (October 4th) and they told me my levels were less than 2. Could I have just implanted late? I haven’t taken a hpt since Tuesday morning, and it was negative. what do I do? I want to test again today but I don’t want to waste money just to get another negative. Would it show up on a home pregnancy test if I was pregnant? What would my HCG levels be now? Please help.