Toddler feels need to poo VERY frequently but then changes her mind..

[MrsRB]

My 2.5 year old girl has been fine learning to wee in the toilet and is happy to poo in the loo too!! However, in the run up to having a poo (sometimes as much as 24hrs before event) constantly asks, sometimes quite desperately, to go to the toilet, I get her there and she doesn't need. It's so frustrating and I'm not sure what to do. Someone had suggested it may be attention seeking (we also have an 8month old baby) but she seems so desperate and genuine.
Any suggestions gratefully received.
Thanks!

How long has she been toilet trained for? My DD is the same age, toilet trained for about 3 weeks now (poos are a bit hit and miss) and she does this a lot. I think it is just them not being quite sure if it is due to come out or not! We get a lot of running to the potty then changing her mind saying it was just a windy pop!