Late Onset PPD?

Does anyone have any experience with this? My son just turned 7 months, and I love him more than the world, but I've been so sad, exhausted, upset lately. He hasn't been sleeping, he's not eating well suddenly, he's been sick 3 times now, I'm constantly worried that he's going to get sick again. I wanted to switch him to formula in hopes that not breastfeeding may cheer me up, but now he fights nursing, but the formula still gives him a bellyache if it's more than 4-6 oz or late at night, so I have to pump him a bottle to make sure he eats enough, and it takes me days to pump a bottles worth. I've taken him to the doctor, the er, the chiropractor, everyone tells me he's fine. Everyone I talk to says "welcome to parenthood" but I just don't feel right. My husband helps so much, and my mom comes twice a week to clean and help me with the baby and I still dread the two or three days I'm alone with him. I don't want to go back to work, I don't know how I could, I don't sleep at all and my baby is terrified of everyone. I feel like I'm a horrible mother because I can't teach him or play with him because I'm so exhausted and sad all the time, he's always grumpy and tired, and so am I. I feel like such a bad mom because I'm home all day but I never get anything done. I see moms who work full time and have no help from the fathers and do more than I do. I get so excited every chance I have to get away from my baby and I feel so guilty. I never felt like this after he was born. It wasn't until the second time he got sick that I started dreading every morning and shuddering every time he cried. Is this just how all moms feel all the time? Am I just that bad at parenting? I've been telling hubby and both of our families about how I'm feeling and they all just keep saying "get used to it. That's how life is with a kid" and that makes me feel even worse 😔