Rocco is about 22 lbs and 3.5 months old now and I'v experienced the same thing just this past weekend. He was with a 15 yo dachsund and a 2 yr old jack russel/chihuahua mix. The dachsund minds his own business but Rocco kept going after him and antagonizing him and nipping at him so there is a case where my dog did the nipping and I had to keep separating them. The chihuahua/jack russell mix played alot with Rocco but at some points, he would nip at Rocco. But Rocco kept going back for more which I couldn't understand. I guess he's a glutton for punishment lol. I am assuming that this is normal but I am curious myself to see who chimes in. At least you know you're not alone!