What do I do about my son's strange behavior?

I have a 2 year old son [who will be turning 3 in April of next year] and recently he has been displaying some disturbing and strange behavior. My son is usually with me all day everyday. His father isn't present in his nor my life. I've been dating someone for about 6 months now and my son acts out when the three of us are together. Most times he plays with my boyfriend and everything is ok but at other times, he 'll just stand in a corner with a scowl on his face and won't respond to anything or anyone. He sometimes does this when it's just me and him also. He has fits every now and then which consist of hitting things when he gets angry, throwing things, and screaming loudly. I don't know what to do or what could be the reason for this but I am very concerned. Any help, advice and/or suggestions would be greatly appreciated.