4 months after I'm still asking myself questions

I've posted on here before about my mum passing away in November from ovarian cancer. 

I keep thinking about how after chemo therapy everything just went down hill for my mum. 

She only had one session, what I want to know is can 1 session of chemo have a big impact on someone? Like can it make someone worse just from 1? Or would of not made a difference 

sometimes I just feel like maybe if my mum didn't have chemo at all she would of been around longer. 

Plus my mum had lung problems already so I think to myself why would they even give her chemo if she was poorly?

thank you in advance.