It can be. But it doesn't have to be, though, and creating a narrative that it is a mental illness for everybody would not be helpful, both in putting people off seeking help and in the ways they are seen or treated by others.
I don't want to pry or ask for details and I certainly can't speak for you and your experiences, but I could give an example where somebody has been bereaved and developed CPTSD as a result of the experience in part due to not being allowed to take time out, get some help and adjust without it becoming a harder to treat condition/illness. Or that fear could be a symptom of their CPTSD or PTSD rather than as a direct result of the specific experiences that let to that condition; treating a 'simple' case of finding things temporarily difficult following a bereavement the same in words, attitudes and diagnosis as the experiences of somebody who has experienced something that has led to CPTSD and their particular symptoms and presentation does both a disservice.
Making it less personal, as I don't want to come across in any way that could hurt you or denigrate anything you have experienced or are suffering, I'll give an example from my experience;
I have adenomyosis. At my first gynae appointment, I was bleeding profusely (had been for six weeks at that point) and felt absolutely awful, in pain, exhausted and to be told by the trainee doctor that the male lead consultant had seen me walk in and said I was too fat for any treatment so they weren't going to investigate for endometriosis around my ligaments and due to my age it'll probably stop soon anyhow. I felt like crap, the way this faceless male consultant had seen my bloated abdomen and assumed that I was simply old, fat and miserable rather than in pain, anaemic and bleeding like a stuck pig made me feel worse - but I was not depressed.
In the letter to the GP, they informed her that I was depressed and discharged me. I then had to sit through a mood questionnaire and argue with them that no, I did not need antidepressants or a referral for counselling, I needed to be re-referred for Mirena insertion at the very least (as the GP and local clinic were not able to perform the procedure when I was bleeding profusely).
It took another 6 weeks for the bleeding to ease up after insertion and it's still happening 3 months later, but with the reduction in bleeding and swelling, some heavy duty supplementation and spending a hell of a lot of money on things to do with the physical side of so much blood loss and pelvic/abdominal/spinal pain over months, I do not feel anywhere near as awful as I did during that appointment. If I had an appointment today, I would either not be described as depressed in the first place or if it were suggested, I'd be a whole lot more able to point out that fact.
What helped aside from the actual useful hormonal treatment, supplementation, food, acupuncture and the like was two days off work following insertion where I didn't have to worry about whether I'd leak through pads, disposable period pants and clothes with a uterus feeling like a red hot cannonball trying to crawl out via my spine (plus a vaginal vault that really didn't appreciate being injected with anaesthetic multiple times) and instead sit on a quadruple bath sheet on the sofa feeling sorry for myself - and getting copious cups of tea and some sympathy from DP when he came in - without having to pretend to be a functioning human being for 48 hours. That wasn't taking time off due to depression or mental illness, if it came to it, I could have dragged myself into work, been next to useless and not performed adequately the following week either - but it did benefit my mental health to do so.
Had DP not been there, maybe I would have benefited from somebody else just taking (paid) time to hear me complain about how shit I felt and make the right noises so I felt like at least somebody heard me and wasn't dismissing it all as the miserable ramblings of a fat, depressed old woman. That would come under mental health, not mental illness.