I have a female lab Sophie that just turned 7. In the last year it seems like she tires easily. I assumed it was because this summer was really hot and she can’t tolerate the heat. But even on cool days she walks really slow and sometimes she won’t move. She’ll play with her friend biting each other but tires out quickly. My other lab Aggie lived to be 14 and was still walking fast and more energetic. I had her on glucosamin and it made her very lethargic so I took her off if it. I had Sophie on Glycoflex and I thought maybe that was making her lethargic too so I quit giving it to her but it didn’t seem to change anything. Is this normal for a 7 year old dog?