There could be a number of things going on here. As the previous poster said, you may just be very introverted and will need time alone from people after a while. Also, it's possible that you have sensory issues that you haven't recognized yet and you may get angry in reaction to a certian sound, smell, or touch, or that stress could build up over time from previous events in your day or week and you just haven't noticed it.
Here's something you can try: ask your family, friends and boyfriend if they have noticed anything in common with the times you get angry. It could be something happening around them, something they said or did, or previous events leading up to them. If you identify why you get angry, perhaps you could then find ways to keep your anger from happening.