i went through the exact same thing 2 years ago at a conference in Singapore. Lots of students and lecturers. My presentation was fine (I had it all written down). But, like you, my mind went totally blank when asked questions. Totally. I could not answer a single question, there was just a stunned silence in the room, the main lecturer actually had to help me understand the questions cos i was so nervous.

What made it unbearable was that every other student's presentation and their 'Question Time' at the end went perfectly. They all sounded confident, well spoken, perfect. I felt like such an outsider, it was horrible everbody giving me the 'pity' look.

It really was the worst day of my life. But, 2 years on, upon reflection I'm beginning to accept that it was my very first presentation, so it was always going to be flawed. Pity I did not accept this sooner as I ended up taking leave from my PhD a couple of months later because i was so overwhelmed by such situations, being surrounded by amazing professors etc. I may go back in the future if i sort out my confidence issue. But still the thought of answering questions after a presentation terrifies me- what a nightmare!!

I'm also somewhat annoyed at my supervisor because we were preparing anticipated questions. She assured me that we have covered everything, I'm going to do great etc. We have actually spent hours doing this.

It turned out she didn't really anticipate 90% of the questions asked.

I'm not sure you can anticipate questions...it is all about confidence,and there is some advice on the other thread. I've felt exactly like you, I have lost a lot of self confidence on my PhD and still feel I'm not good enough-even after finishing it! I've decided academia isn't for me, but talks are something that takes practice.
Do you talk to your lab? You could do with a number of people asking you questions in a safer situation as they will all have different perspectives and knowledge.

I have experienced that sort of thing as well. When i talked to my supervisor about it afterwards, she simply said:
'you should probably get more practice at being able to speak in stresful situations... you dont want to be at a loss for your viva do you...'