"So, not to ask a dumb question, but if my child reaches both these things before the age of one, do I need to buy another seat?? I'm so confused. What if she weighs more than 22 lbs but is shorter than 29"?"
There's no such thing as a dumb question when it comes to our babies!

To answer your question though, I think it's one or the other. If she's reached the weight limit but not the height limit (or visa-versa), it's time for a new seat. We bought a convertible car seat when our son was 7 or 8 months. It can be put rear facing 'til a year and then be switched to forward facing. I would highly recommend one like that. They last quite awhile, I think our's goes to 40 lbs. Aden is now 14.5 months and he just loves it.