Hello Juliesmith, and a warm welcome to the site.
It's lovely to be concerned about your parents, especially as we get older in life, unfortunately, this is unavoidable and we never believe it will ever happen, but it does as the years pass us so quickly.
If I could ask you to type this in your browser or the search bar at the top of this page 'intrusive thoughts', then different remarks from many people may explain what you may be suffering from, as I'm not a doctor I can't diagnose you, but a doctor will be able to do this for you, but I don't want you to worry about this as I too have OCD and had these type of thoughts many times.
As I've had this condition for about 60 years I know that these thoughts don't actually come true, but this doesn't stop us from having them and it's so difficult trying to explain to someone else what we are thinking about and our reasons why, because they don't believe us, but we still have them.
I understand what you have said, so if you could read a few comments made by different people then this may help you, and please come back and ask any questions you like, many of us will be able to answer them for you.