If you used your 18-55 you would have been able to get more, but you probably wouldn't have been able to open up your aperture enough. I went through the same thing on a new baby shoot using a 50 mm on a crop body and only natural light. This led me to the conclusion that I needed the 24-70 2.8 lens. Not only did I buy that, but I bought a full frame camera which would have worked fine with the 50 mm lens.
Camera: Canon EOS Digital Rebel XTi
Exposure: 0.01 sec (1/100)
Aperture: f/5.0
Focal Length: 42 mm
Exposure: +0.65
ISO Speed: 400
__________________
Lori Putman flickr
~No one can drive us crazy unless we give them the keys
~~Life isn't about waiting for the storm to pass, it's about learning to dance in the rain!
7D | 300L f/4 IS | 135L | 35L | 100/2.0 | 50/1.4
430 EX, 580 EX II Speedlites