11 mo. old not eating (and we just found part of a tooth)

I may be overreacting a little here, but I just wanted to see if anyone else had ever experienced this with their pup. My husband and I just moved a couple days ago so we figured our little 11 mo. Havanese was not eating b/c of stress, etc. However, when we would give her treats that she used to love (we tried 2 kinds) she would try to chew on them and then spit it out as if she was having trouble chewing (little Bil-Jac treats, soft pieces). Somehow she managed to eat her food last night and this morning (small kibble) and now we found what looks like a piece of tooth on the floor which she was playing with. Since I thought it might be a part of a tooth and she seemed to have chewing issues I looked inside her mouth and noticed that she doesn't appear to have ANY molars on the upper left side of her mouth. She had to have 2 teeth pulled when she was spayed b/c they wouldn't come out on their own so I don't know if this contributed to it. I know I should be brushing her teeth and could have noticed this sooner, but now I don't know if she is actually losing teeth or if they never grew back in. Sorry about the rambling, I just wanted to give a good description of what has been going on and would love any input on this!

I think you should take her to the vet, she might have some infections that makes the teeth sore when she eats.