Those of us trained in mathematics tend to interpret words like "multiply" as if a mathematical problem was being stated. However, making an argument of this sort on the basis of the word רָבָה, to cause to increase, would be a stretch. Remember that even the root meaning of the English word multiply is to make many. Given the context, the emphasis on this verse is that she has plunged herself into sorrow when she was in bliss before, not she had pain and now it would be increased.
On the other hand, it is notable that the line between pain and a strong pleasure is not distinct. For example, I enjoy a level of spiciness in my food that many people would consider painful; but there are others to whom the level of spice which I can tolerate does not seem painful at all. It is purely in the realm of speculation to try to pin down exactly what type of sensory experiences there might have been prelapsarian. A weak pain may be a pleasure, where a multiplied pain is agony.
The deciding issue is really the context. She has passed over from a state of beatitude into one that is accursed. Regardless of the exact meaning of pain, sorrow clearly would not exist in a world pronounced so profoundly good (Gen 1:4, 10, 12, 18, 21, 25, 31). The point of the narrative is to show that the blame for the sorrows of the human race rests squarely on its own shoulders.