Faint test with twin pregnancy?

Hey this might sound like the silliest question, but as a first time pregnancy I’m so lost on what’s normal and what isn’t! So I tested positive at the start of July, I was testing regularly for the first couple of weeks because I couldn’t believe it and had a few “dye stealers”, then had some cramping and bleeding at the end of July when I was around 6w. I went to the EPU and had an early scan which revealed fraternal twins and we saw both hearts beating which was lovely! Now for the silly bit, I’m now 8w and I found an old unused pregnancy test, and just decided to do it for fun as I thought with having double the baby there’d be double the HCG and I wanted to see how strong the line was, but it came up incredibly faint. I’m now worried that something has happened to drop my HCG even though I still have symptoms otherwise (sore boobs, fatigue, nausea). I know the test was at least a couple of years old as it was from a pack I bought before we moved into our current house, I just found it whilst cleaning the bedroom. Why do you think the test came up so faintly? Could it indicate something is wrong? I saw the midwife for my booking appointment on Friday and she said my urine was fine from the sample but I’m not sure if HCG is something they check. Anything to ease my mind would be lovely😅 thank you in advance! Xx