Feeling like something is wrong

I am 5 weeks pregnant with my third. This baby was so wanted and planned but ever since I got a positive test I am having a hard time getting excited and constantly feeling like something is wrong with the baby. This has now progressed to fear of a miscarriage. I started out with a ton of pregnancy symptoms that have slowly gotten better to almost gone over the last week. I barely even have an appetite. I’ve been laying around all weekend feeling mopey and just trying to connect and I can’t. This would be our last baby so maybe that is it? Has this happened to anyone else?