why would she do this?

we co-sleep with my DD and she wakes up after 30 minutes of being in bed asleep with us for a booby feed, but will then go straight back to sleep for a few hours. why do you think she is doing this? it means i can't start her off in her own cot because it will only last 30 minutes before she needs me again anyway. also the last 2 nights she has been waking up after this 30 minute period but is then wanting to play and babble and roll around, try to crawl etc. it is very frustrating! it takes then an hour or so to get her back to sleep. i don't really know why she has started this either. any ideas or suggestions to get her out of these habits?

oh, i forgot to mention that she is 5 months almost 6 months.

thanks for that, it always helps to know that others are going through the same thing.