why

Help me understand this..... Are people who are narcs bipolar also? I ask because I find that my boyfriend is "normal" one day and the next day will wake up and be back to his old narc self.

For example: Friday was a horrible day. I woke up knowing something was wrong. He wouldn't speak much and I kept asking what was wrong and he wouldn't talk. I knew in the hours to come he would text me with the problem. Sure enough-- he did. He asked why I was talking to one of his ex wives and a girlfriend of his friend on the phone. He checked my phone and saw a call to each of them. Once again, if he would have did all his investigating he was wrong again. The call to his ex he made me make one night when he was in a rage that I had a "thing" for a family friend. The other call was meaningless also. He wouldn't let it go. Accuasations started flying about things he assumed I did in my past before I was with him that wasn't true. I cried and cried and begged him to stop and tried so hard to proove to him none of it was true but he has these thoughts in his head and they won't leave. No matter what I would say he wouldn't believe it. The night got worse and worse. He continued with horrible words and wouldn't stop. All I could do was cry. We even got into a shoving episode. It was disgraceful.
The next day it was like he was a new man. He woke up and you would have thought that we had the perfect relationship. He was loving, kind, appreciative. I didn't understand it and couldn't question it because I knew where it would go if I did. This continued for 2 days. Of course, this is after I bought us dinner, lunch, groceries and gas for him too. He pays for nothing and lives at my house for FREE.
Tonight he walks in the door and I am cooking spaghetti and have a cake in the oven. He says, "I thought we were having a casserole, I was in the mood for that." I said, "I'll fix that this week I had the stuff for this." He ate it and seemed to enjoy it then I asked him if he wanted cake. He said, "I don't like chocolate cake and I am not eating that." I said I would take the icing off--and did. He said he wasn't eating that crumbled up cake. I took the plate brought it over to the trash and threw it away and the whole cake too. He walked away and told me to leave him alone.
I confronted him and he ignored me and told me to get out of the way. I asked him why couldn't he just have tried it or acted like he appreciated my efforts or even apologize. He said I don't like it why am I going to eat it? I cried and he told me I was crazy. He brought up an incident from yesterday and said, "Go find someone else who says what you want them to." I asked him why he had to be so mean and disrespectful and he said he wasn't-- just to leave him alone. I walked away crying.

Am I overreacting?

How do you go from being so kind one day and so mean the next? What is it with this man?