Try not to come up with reasons for anything, (like long covid etc) let the doctor diagnose.I have lost some weight, but not a huge amount. Probably because I'm eating less tbh. No one mentioned thyroid so I might ask the dr.
Weight loss is a red flag symptom as is new onset pain or weakness. which they will definitely investigate more. If pain worsens in the evening that’s also significant.
Make a list of all symptoms and speak to the doctor again omitting anything that you think could explain it. You want them to work for you rather than accept a self diagnosis and get you out of the door. You have to really push.
Blood tests are only relevant for the current time so the previous blood test is a marker but not relevant to how you’re feeling now.
I’m not saying this to worry you, it sounds a lot like long covid but just let the doctors decide so you know for sure otherwise the anxiety of ‘is it though’ will make you more poorly.