So, when did you get your MD? Priapetic erections can damage you PERMANENTLY:
link. Granted, that article specifically says that Viagra is unlikely to cause trouble, but if you don't need a med, and take it anyway especially not knowing what might conflict with it, you take a real chance with your health. How'd you like to have to have a PERMANENT loss of erection, requiring some other kind of fix, such as a penile implant? Sound like fun?
Look, medicine is not an area to fuck around with. Even trained MDs fuck up more often than they like. And you took it w/o regard for its contraindictions, w/o any kind of support or advice. I just hope, for your sake, that you haven't harmed yourself.
If you want to stimulate your woman longer than you can with your own penis, get a dildo and use it on her. It's much safer, doesn't require a prescription, and you can use different ones with all sorts of features. And it's safer. Did I mention that it's safer? I just wanted to make sure I mentioned that it's safer.