You're having what's called intrusive thoughts. They are a symptom of post partum depression (PPD), and also of obsessive-compulsive disorder (OCD).
Thing is, its not me. Hubby has worked nights throughout our whole relationship but this only started after having DS and i only fear, for DS... not me. I never have thoughts of anyone doing anything to me, just him.
This makes sense. You probably developed a type of PPD after you had your son - which is why you didn't have the thoughts until he was on the scene, and why the thoughts centre on him and his safety.
They go as quick as they come and im left horrified of what i just thought.
It's actually normal to have intrusive thoughts - pretty much everyone has them. The part that isn't normal is that they sort of get stuck in your head and really upset you/ cause you to suffer. Most folk dismiss their intrusive thoughts so quickly that they may not even be able to remember they had them, that's all.
Basically, if you were to go to therapy for this, what they'd do is just teach you to dismiss the thoughts. It's a skill you need to develop. That, and they'd teach you skills to soothe yourself once you get upset. This would all be part of something called "cognitive behavioural therapy" (CBT) - the "cognitive" part is about learning how your thoughts work, and the "behavioural" part is about learning to change your behaviour in order to control/lessen the thoughts.
Medication can also help a lot - like looooads and loads. Definitely talk to GP when you can. But please do know that the best thing to do, is to have therapy (including accessing therapy online! doesn't have to be in person) and also take medication. The combination is very powerful and is likely to relieve your suffering.
Wishing you the very best. What a shitty thing you have been through!!