My 1 and a half year old is having problems with her bowels......?

she will be 2 this may. this has been a problem for about 8 months now. she finds it difficult to p*o and sometimes will keep it in for days at a time, so when she can finally go it is worse than something from a grown adult and obviously ecrusiatingly painfull for her... how could i help ease this for my little girl??