I had a very similar situation recently where I flopped middle pair out of the BB against the small blind.. he bet out, I raised.. he raised all-in for his last bit.. I had him covered and by that time I was too deep into the hand to turn back (I think I was getting like 4 or 5:1, something big).. He had top pair and it held up.. I am a very stubborn player in heads-up or SB vs. BB situations