Discussions

We have had our cat for close to two years now and he has always been great. But the past couple days hes been acting strange. We just got a puppy and at first he would just watch her were ever she went. Then he started chasing her (not in a mean way) and they would play. But now the last couple days hes getting "Too Friendly" He pins her to the ground and tries to do his thing. We stop him and tell him no but he keeps doing it.  
 
Could it just be because hes jealous? Or maybe hes all twiterpated because its spring lol. I really have no idea. Please help!