horses change shape all the time, my mares saddle is a little tight on her at the moment as shes a bit on the podgy side, its not so tight it rubs her or hurts her.  to make sure you have the saddle in the right place put it on there neck and slide it down to there back until it stops and that should be the correct place. if you cant get the side of your hand down the front of the saddle then its to tight. :-) if she has changed shape place a pressure  pad underneth the saddle to raise it up abit......... or to be on the safe side get a proper saddle fitter out as a ill fitting saddle can do your horses back alot of damage

Hey if i were you id get a saddle fitter in to check it fits correctly, over the few months you have got her she could of gained more muscle which may explain why it fitted before? If it were me id get her tack checked because if you leave an incorrect saddle fit for too long it can cause quite alot of problems such as acting differently when riding because its hurting her etc. hope i helped xx