Yeah I suffer badly from this too. I've always been told its another symptom of my OCD but health anxiety/hypochondria can exist on their own without any underlying condition like OCD.
I've had it ever since my OCD first developed, so since about aged 5ish. I got therapy for that and OCD as a kid and then again throughout my teens and adult life but its stuck. I have lost count how many illnesses I have diagnosed myself with and so far, all predictions I've came up with have been wrong but this still doesn't settle my nerves and make me realise its just my anxiety playing up, not my body being in any of kind of health danger. I check my body almost every minute of the day for signs of illness, I even take pictures of my body to compare areas day to day at times, especially recently when I had a fungal infection on my scalp. It sounds extreme but that is what OCD and/or health anxiety makes you do.
I have this every single day to some extent. One day, it could be my swollen lymph nodes that have been swollen for years that convince me I have lymphoma even though they have been checked numerous times by the doctor. The next day it could be heart worries because I keep getting chest pains and palpitations even though I know they are caused by anxiety and stress. The next day I could have a migraine like I've had a million times in the past but I'll think its a stroke or brain tumour. No amount of logic can get rid of this. The more you try and use logic to try and show yourself that you are fine, the further deep you bury yourself in it feels like. You have to wait to come out of the worry on your own and that could take hours, days, weeks, months.
Its hell. It makes you feel not safe in your own body but not brave enough to go and get checked over in case there is actually anything wrong so you live in a continuous loop over worrying what could be wrong but too scared to do anything about it.