H.G.
It may be a combo of hormone changes and brain overload. Years ago, my DH had severe health issues. He couldn't work. I was carrying us financially and doing just about everything around the house plus looking after his medical stuff and working full time. I remember confiding to our chiropractor at the time (in tears) that I couldn't put a sentence together most days. It was scary and I thought I was losing my mind. He explained that when you're brain is overloaded (with work, worry, or just tasks like keeping that new baby happy and healthy), something has to give and often it's your memory and/or speech.
Taking care of a new baby is a lot of work. You're probably sleep deprived or you just may not be sleeping soundly like you used to because you're listening for the baby or waking for feedings, etc. Give yourself a break and try to relax and ask for help if you need it. It gets easier down the road. You just have a case of "Mommy brain" for the moment.