I felt like this when it seemed like all of the responsibilities of the family (shopping, cleaning, caring for DD...) were mine. When I went back to work and DH did more, it got better for me. You didn't mention if you were a working or stay at home mom... For me it was needing a break. I was 36 when married and DD was born and not used to being so totally tied down at home, etc... It was a real adjustment for me. I totally LOVE being a mom, but breaks are really necessary!
Perhaps you could explore when you feel like this and how exactly you feel. I know that the smallest, dumbest thing could set me off... a pity party of sorts.
Of course, many of the other suggestions are great. You sure have a lot to think about and explore. Good luck. It will be best to look into this soon so that your son doesn't pick up on it and incorporate it into his behavior... cheers to you for recognizing it and wanting to take action.