Me and my boyfriend have been going out for about eight months. We started to be intimate, but before we started to have intercourse he got up to get a condom. I saw him put it on. He kept slipping out when we started; I stopped him because it hurt and when I grabbed him I realized he wasn't wearing the condom anymore. I was so upset and he just said " I thought you knew I took it off." After that he said sorry and that he thought I wouldn't mind. After that I just went home and haven't spoken to him. Now I'm worried about the possibillity of getting pregnant. It's been three hours since I got home and my stomach has started to hurt a little bit. He didn't ejaculate and we didn't get far because he kept slipping, but I don't know. I've heard of precum causing pregnancy, but intercourse didn't go further than a few shallow thrusts. I'm going to go get a morning after pill tomorrow. I honestly can't believe it , I don't know what to do about the relationship now...

How old are you two. Sounds like someone either didn't know what he was doing or he was trying to go against your wishes and have sex without a condom.
I suggest you leave sex alone for a while and certainly with him.

I'm 21 years old. He sent me a message this morning and said he knew that he was wrong for that, but I was overreacting.

You are not over reacting and he was wrong. At his age he ought to know better. Leave him alone. Does not sound like a person who is sorry or respectful.