I am so sorry that you're experiencing these symptoms - I've experienced them, myself to the point where I was taken to the hospital, by ambulance, because I thought I was having heart attacks. This happened several times.
It sounds as if you are experiencing anxiety disorder - I am NOT a professional, by any stretch of the imagination, but I suffer from this condition, myself, and I have spent years learning to manage it.
From your description, I am guessing that both you and your fiance are living with your mother? Is your father involved in your life, at all? I didn't see any mention of him in your description. You have a new baby? Your first? Is your fiance the father of this child? Any symptoms of post-partum depression? Are you working, at the same time? Going to school?
I would strongly encourage you to actively seek some counseling therapy to find the root of this anxiety, process whatever the events were that contributed, and learn how to manage it. Anxiety can absolutely cause people to think all sorts of things and alter our own beliefs and perceptions about what's going on in the world outside of our heads.
Anxiety also creates an unstoppable vortex of panic. This happens, that happens, the other thing happens, and the anxiety spins into panic, the panic into depression, the depression into more anxiety, and so on.
There are steps that we can take to manage anxiety and learn to calm our minds. This does NOT happen, overnight. This requires time, patience, practice, and courage. And, it's an ongoing thing - it is something that I will be managing for the rest of my life. But, each day isn't as challenging as the previous one. And, when "Life" happens (unexpected catastrophe), I am SO better able to deal with what's going on rather than build stories in my head about what's going to happen down the road.
Also - if the dynamics between you, your mother, your fiance and all three of you, at once, is negative and/or abusive, that, alone, can cause physical symptoms that mimic poisoning or chronic illnesses. I know this from personal experience. People (particularly women) will develop physical symptoms, chronic illneses, acute conditions, and medical diseases if they experience long-term abuse or trauma. So, you're not losing your mind, you're not crazy, and you ARE worthy of getting help for yourself.
Brightest blessings of comfort and encouragement to you.