You are not on your own, I don't know how to advise so I'll tell you my story.
I am a very calm, happy person, I hardly ever used to loose my temper.
When I was pregnant with dd I blamed the hormones for making me feel crazy. When dd was 6 months old I blamed the lack of sleep for what was becoming a terrible anger. When dd was 9 months old my mother was reading the paper about some woman who had hurt her kids, she said "How could anyone be so evil". I understood how that woman felt, my anger had become so bad, I had to hold myself back from hurting my dd. It's a painful thing to admit but thats the truth. I went to the doctor who prescribed ad's and I went for CBT. The ad's did help, the CBT maybe helped the way I think but it was still there. Dd stopped feeding from me at 18months and my anger went with it and I stopped the ad's. I truely believe it was the hormones or a chemical imbalance as my hv calls it.
When I got pregnant with ds I felt it building again so soon after he was born I went back on the pills. I think it's better to bf and be on ad's than to not bf at all. I enjoy doing it and I love the bond I have with ds. He's 11mo and showing no signs of cutting down feeding so we'll just go until he's ready to stop.
Anger is part of pnd but it's not as common as feeling down. My hv explains it as a chemical imbalance that just happens sometimes, she joked that I'll go even more mental when the monopause starts.
With you it could be this, or it could be sleep deprivation or pnd.
If it gets to the point that you feel you are struggling to control it, go to the doctor. You don't have to get ad's there are other options. I found ringing towels in the bathroom helped or going into the garden shed and shouting obsenities. Sounds silly but you've got to let it out.
Hope this helps, it's very lonely being angry.