Welcome to HCM.
'This could not happen to us' is a common feeling for every one of us. It may be false sense of security.
As you were together for 15 years, as you knew your partner is HIV
positive, it is better for you to have a HIV screening in your blood. Your partner could be reactive for HIV earlier and he might have come to know about this only recently. So it is better to have a blood check up and rule out the possibility. Even if your partner has HIV earlier, there is a chance for you not getting infected and it is called as Sero- Discordant stage. So nothing to worry.
good luck to you.