I had lots of intrusive thoughts like these when DD was born, and the HV explained that it was down to a combination of things - suddenly you are completely responsible for a tiny scrap of a human, without any training (and way too much awareness of what can go wrong), and your hormones being all over the place.
Our hormones control parts of our brains as well as our reproductive systems (which is why anxiety can be so debilitating during the menopause), add in lack of sleep, and the sudden huge love and protectiveness you feel...and no wonder you are overthinking everything.
For me it was "would that picture that's been on the wall for 10 years suddenly fall onto the baby?", "what if I accidently cut the baby when I was using scissors (nowhere near her)", "what if she's too hot/cold and I can't tell?", "has she breathed in the nail-varnish remover her sister is using in the next room?"...honestly it never stopped.
I was prescribed some anti-anxiety medicine, and the doctor also suggested I use a mantra when it was really bad "thoughts are not facts" and I said it over, and over, and over again.
DD is now a noisy and very lively tween. Get yourself some help lovely, so that you can begin to enjoy your little one xxx