Only 4 weeks and already... very grumpy

I'm afraid by how angry I've been for 2 days... really out of my comfort zone. I have anumber almost 2 yr old son and he and I just had the worst couple days... normally he is a super happy fun kid and now we are not getting along and I'm so angry. I feel terrible about it... anyone have similar mood swings? My first pregnancy was super easy fun and happy times so I'm at a loss here... what the heck is wrong with me??