Welcome to HCM and thanks for asking your question. I can understand your concern.
I would like to enlighten you that usually if you are having 30 day regular cycle, then from 12th to 18th day are considered unsafe and remaining days are relatively safe, because ovulation does occur around 14th day of a cycle.
But believe or not, it will never be 100% safe, because ovulation could have been occur anytime. But if your preg test is negative, then you need not to worry much about it. I would advise you to wait for few more days and if menses does not start, then you need to repeat the pregnancy
test, because many times this test is very lightly positive in early days or you can also go for ultrasound
to rule out pregnancy. Also you are 42 years old and will approach to menopause
within next few years, cycle may become irregular, so don't worry.
Hope this will help you and feel free to answer any of your further relevant queries directly to me anytime.