Too late for post natal depression?

How long after having a child can you be diagnosed with post natal depression. I have a 2yo and I struggled really bad with depression after she was born, I never really sort help for it but thought I was on top of it. Slowly and slowly I feel it's creeping back, I love my daughter so much but I just get so angry and frustrated at her. Her crying annoys me and I over react to normal toddler behaviours, I just feel I don't get joy from her or anything in life at the moment. I don't know what to do or where to go. I'm a single mum and studying at the moment so I literally never get a break. I obviously am not in a position where I'm going to hurt myself or my daughter and never would I just want to start enjoying life again.