DOG HELP PLEASE!

I really need advice, I don't know what to do. I have a teacup yorkie and I've had her since I was 15. She's 9 years old. She still has not adjusted to my son, and I'm having another in September. She potties in the house but ONLY for me (yes she goes out multiple times a day). We had a house/dog sitter all weekend and she said she was great!  No accidents in the house, nothing. Well this morning I take her out like usual and she goes and does her business (I never call her back inside bc I like to let her take her time bc she's a small dog). Anyways she comes back up to me and we go back inside. I get something for my son and go to my bedroom and there is a steaming pile of dog poop in the bedroom doorway (she always poops or pees somewhere she knows I'll see or walk, she's even looked at me while doing it). Lately she's been getting snippy with my son and snapping at him. I try to keep them separated as much as possible, but it's not that easy, he's 18 months and playful. I'm at a loss of what to do. I love her, but I don't know how to get her to adjust better? I still pay attention to her, let her get on the couch and all that. She's been my dog for 9 years so I don't want to get rid of her, that's not even a thought. We just went for a vet visit and she's in perfect health so the accidents aren't an old age related thing. Small dogs live pretty long. I seriously think it's a personal issue bc I had my son. Small dogs have very strong personalities and are extremely smart so I'm pretty positive she knows what she's doing. I just feel so stressed and she's only going to act out more when the baby gets here, any advice would help. Thanks