I was mildly anxious after the event but then my PTSD got worse a couple months after for seemingly no reason?

CW for violence, abuse and drugs

I 26F work in a social services job (an already stressful job where I’m regularly exposed to verbal abuse and threats). Earlier this year, my workplace had me working alone with a teenage client that I quickly realised was high on m*th. He was totally fine at first and in a positive mood but in the span of a couple of minutes his presentation turned. I had to lock myself in a room while he attempted to break inside, threatening to k*ll me. Some other stuff happened but I won’t get into that.
I took 10 days off work due to stress and mild PTSD symptoms (irritability, struggle sleeping, nightmares etc) and then returned to work for 3 days afterwards before going on a one week holiday (planned months in advance). Felt fine on my holiday. After my holiday, I came back to work for 2 weeks and originally felt fine. Some anxiety but manageable for the most part. I continued working in the same space where the incident occurred.
Then suddenly it hit me. I began suffering with vivid flashbacks, I can’t sleep, having panic attacks and nightmares and struggling to leave my house. At night I get anxiety about people breaking into my house. I’m paranoid about people, especially males and teenagers, when in public because I feel like I can’t gauge when someone is a threat and when they could turn on me. I was diagnosed with PTSD.
I’m so confused as to why I was semi-okay for a while but then as time has gone on, it’s gotten worse. Shouldn’t it be the other way around? I feel like an imposter.

TLDR; Took a couple months for PTSD to fully take hold after the event. Is this normal? I feel crazy and like an imposter.