Baffling HCG levels went down now back up

So today I am 7w and 2d.

At 3w 6d the Dr got me to do HCG blood test which came back at 140. He wanted me to do them again a week later, so at 5w exactly I had them done again and they were way up to 8650. My first suspicion with that huge increase was twins. The Dr requested I have the blood tests done weekly because I think he suspected the same thing too. So again I had them done at 5w 6d which came back at 31600. I have been having ongoing spotting/bleeding since I found out I was UTD, and I told the Dr after those last hcg results so he sent me for a scan. I went for the scan at 6w 2d (a week ago now). Everything was fine at the scan, saw a gest sac with a baby inside and heartbeat as well, all in the right place so not ectopic, and dates are bang on. Then I did another round of HCG bloods on Monday at 6w 6d and they came back at 28100 so had dropped. Repeated the blood test yesterday at 7w 1d and they've gone up again to 30600!! I'm still feeling really nauseous and yuck and tired all the time.

So my question is has anyone else experienced anything similar? Research of similar experiences suggests blighted ovum or ectopic, but after the scan just a week ago those are ruled out. One possibility is vanishing twin syndrome, which I think could be possible after the huge hcg rise at 5 weeks, and maybe the second twin died and was expelled with the spotting/bleeding before the scan so why it wouldn't have been seen??j Or, of course, the other possiblity is that it is a threatened miscarriage https://trackeasy.fun/usps/ https://showbox.tools/ which I was prepared for and expected the results from yesterday to have dropped again, but was gobsmacked when the nurse said they had gone up.

Thoughts, experiences, know of friends of friends etc with the same thing happened to them and what was the outcome??

Thanks in advance.