Why has my dog suddenly started snapping?

Okay so my mom and I are both his primary caretakers. He has always been so kind to else and is very protective of us. Lately, he snapped at me once yesterday morning and he snapped at my mom as well today. He isn't biting down but it's kind of a snarl with a fake bite. I'm wondering what the cause of this can be. I did change his food schedule to what a normal dog should eat because he really needs a diet. We used to just refill his bowl whenever it ran out. He is a papillon. I am giving him the recommended food amount of the box (1 1/2 cups per day). I give him about 1/2 cup of food 3 times a day. Is he getting pissed because of this? What's the problem here? He is a 6 year old papillon and has always been a little aggressive but never towards me or my mom.