As with what AShadow said, not all traumatic experiences can cause PTSD. That will depend on the level of severity in the situation as well as the individual persons coping skills.
Look at what kinds of symptoms you have from this, there are a certain amount of symptoms you need to have to be classed as PTSD. For example nightmares, especially relating to the traumatic situation, avoidance (in your situation maybe avoiding situations where knives may be involved, or certain people), triggers and flashbacks (to give you an example because I found it hard to understand what a flashback was when I was initially diagnosed- I was abused my someone who had a temper, an absolutely demonic anger and he'd get in my face and scream. So one of my triggers is any sort of anger or nastiness directed at me, if someone gets mad at me for anything my heart starts pounding, I start shaking, I cry, and if someone gets REALLY mad at me I can get suicidal, and it can all last for up to 2 or 3 days. Of course it's not always that severe. But to try and explain what it is like- you know when you smell something, etc. that reminds you of someone's house from the past, a time in your childhood, and that nostalgia comes over you and you feel like you're there again? A flashback is a bit like that, except, you know, not exactly enjoyable. I hope that makes sense). There is also hypervigilance, so being extremely edgy around someone with a knife (keeping yourself on guard in case they might suddenly attack you).

These are a few symptoms of PTSD,it's not impossible that you have it, as you would have felt as though you're life was at risk. It is definitely something you should talk to a T about, they can tell you all the symptoms and discuss how you feel about what happened.