This could be from viral disease such as calici virus or herpes virus which you can read about here
www.marvistavet.com/html/feline_upper_respiratory.html
If the cat is having a hard time eating then softening its food may help or feeding canned food.
The sores could be from periodontal disease or from a senstivity to the food being fed. Sometimes changing to a higher quality than grocery store type food can help with that.
If you feel you need to help your pet now with a vet visit you might want to check www.carecredit.com to see if you can finance the vet visit and pay it off over time.
Hope this helps you!
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