Hiya deedi,
Have you found that your mum's tastes have changed and that she will now choose sweet food over savoury food? This could be why she is eating little. Another thing to rule out is whether there is any problem with her mouth. Does she have oral thrush perhaps or dentures are rubbing her gums, both of which can be painful and stop them eating. Maybe try her on sweet stuff for a while as they can be loaded with calories which is your main aim at the moment. Sponge and custard (made with extra cream in etc.), or home made rice pudding, tapioca, bread and butter pudding, cakes, biscuits, meringue nests with cream and diced fruits, little tubs of jelly, ice cream can all tempt them and help the appetite. An old savoury life saver is mashed potato and gravy. Can add butter and cream and even an egg to the potato and lots of nutrition if you make up batches of your own gravy.
Also, watch when your mum is eating and make sure she isn't having problems with swallowing. If you feel this might be the problem then ask the GP for an urgent referral to a SALT.. Speech and Linguistic Therapist. They have the ability to test for this and can offer solutions eg thickened drinks etc.
Hope this helps,
Fiona