Why is my 3-y-o so negative lately?

[Asparagus78]

The negativity is driving me insane! From the moment DS wakes up till he falls asleep, he acts as thought there's a dark cloud over his head and he simply cannot snap out of it. For example, this morning he woke up screaming mad that the door to his room was closed (yesterday he had a meltdown because it was open and claims we woke him up...). Then he didn't want any of us to have breakfast and started screaming when I sat down to eat. When asked what he wants to do or eat or whatever, he replies "nothing" or yells "don't talk to me!". On Friday, I fought with him to get him to playgroup - almost 2 hours of screaming and arguing until we finally got there and he agreed to go in - and he had a blast. But when he "caught" me smiling at him (he was playing trucks with another little boy and they were laughing), he started sulking and said he hated teh place and wanted to go home. Same thing tonight - he was listening to a CD and singing along, when DH chimed in and said he thought singing was stupid and he wanted to break the CD player. These are just examples - I could pick any moment of the day (he acts like that over snacks, mealtimes, bath time, playtime, bed time, with friends, family, his little brother...) What is going on here? This behaviour started a few weeks ago, but I can't pinpoint an exact moment. He doesn't go to preschool and pretty much stays home with me all day since going anywhere starts hour-long fights, although I do insist on outside play everyday. I thought he might be bored, but then challenges every offer of activity (if I suggest the playground, or the library, or ask him what HE wants to do, I get "NO" and ''Nothing!" as an answer). Please tell me this is just a phase...