yes its normal, because u did spend quite a while with her before so yer still "attached" and when u hear she has a new bf, yer just mesmerizing the memories u guys had together thats all
reminiscing is what ur doing i guess... and yes it is normal...u just need to sort out ur feelings thats all...i'm sure u love ur ex but dont let it affect ur relationship with ur current gf...cos its not fair on her.. but all the best...