You're not doing anything wrong! Good job on catching the sleepy cues early--it took me until about 9 weeks to realize my son was tired after only 45 min. of waking. The 40-45 minute nap is normal at this age. If you read around, you'll find that babies don't figure out day sleep until 3-6 months. If you can get her back down great, if not, get her up for a little activity and put her down again at the next sleepy cue. I know it cuts into your time--wouldn't it be nice to have a 2-hour nap break!--but just let her get the sleep she needs when she needs it. She'll figure out longer naps when she can start staying up longer.
Hope that helps!
Melissa