Numb mouth

So, I LOVE going down on my bf, but every time that I do it for over 3 minutes, my mouth starts getting numb and I can't feel it. I feelike when you go to the dentist and they put anesthesia to not feel pain, like if you drink water it's gonna slip right out of your mouth haha. And it's always before he cums, so I don't think I might be allergic to that, what do you think