Hi there,
First, 49 is a good score for math, considering what people have been saying. I took the GMAT last month and ran out of time during math. I scored a 47 after I wildly guessed the last five questions. I thought I had done much much worse. I have also heard that leaving a question blank counts more against you than filling it in. But by how much, I am not sure!!
I believe that the first portion of the test counts much more than the last part. I think (I could be wrong) that you may score the most 1 pt on your last question, so it would bring it up to 50, not 51. However, the way ETS does its tricks, I doubt that anyone really can figure know for sure. For your last question might have been an experimental one!!!