Why Is My Dog Suddenly Acting Aggressive?

I live with my boyfriend and we have a 1yr old Papillon male. He is extremely friendly with all people and animals he encounters. He has had a few instances where he has shown aggression when trying to take food from him but this hasnt happened since he was a lot younger. In the last month or so he has snarled at my boyfriend twice and tried to bite him. Both of these times have been when the dog is sitting or laying right next to me and my boyfriend is trying to pick him up. It seems to me that he is being possessive of me and is trying to bite so that he is not taken from me, even just to go on a walk. Any other time the dog will run up to my boyfriend and greet him enthusiastically and he clearly cares for him a great deal, but I am concerned by this sudden change in his behavior. Our dog is crate trained and we have a young kitten as well that he treats like a loved brother. Overnight the dog sleeps in his crate and during the day he is left out around the house. He has never had professional training but knows “sit, stay, get down, lay down, get the ball, and drop it”.

Dog Acting Crazy When I Try And Train Him?

When I first started training my little Papillon, he was really good. It only took an hour and a half and he learned to shake paw, to lay down and to come here. I started out by giving him a small treat when he did what I asked, and then eventually stopped giving him treats and he still did what I asked. I worked with him for a couple months and now he’s got those commands perfect.
I recently started trying to teach him to sit. The first time I asked him to sit, he did it, but he got really upset. He started shaking like crazy and he ran away from me and hid behind the toilet. After I got him out from behind the toilet, he wouldn’t come near me for the rest of the night. I ask him to sit like once a day, I thought maybe once he got used to the word, and used to doing it then he would stop being afraid, but he hasn’t. He knows how to sit, but I have to ask him to sit 50 million times before he will do it because he is so upset and trying to get away from me. He won’t even take a treat after he sits, and he doesn’t care how much I praise him after he sits. After he runs away, I tell him to come here and he does, and then I ask him to sit until he does. He acts like I’m hurting him by trying to get him to sit. After he finally sits, I tell him to lay down or shake a paw, but by that point he’s so upset and wants nothing to do with me. And btw, I’ve never hurt him. He’s never been hurt by me. I’ve had him for like 7 months and he’s definitely used to me. He’s like my baby and always wants to be around me, so it confuses me that it scares him so much that he’d want to run away from me. And it’s not just me, he will run away from my fiance if he asks him to sit and he runs to me.
I got him from a shelter and he was a puppy mill dog. Could something have happened to him at the puppy mill that would make him afraid to sit?? Any ideas why his behavior has suddenly changed?