Are you underweight, or are you under a lot of stress? Women's ovulation (and therefore their periods) often go AWOL if that is the case.
I'd ask the doc to do a hormone test. For what it is worth, cancer does not usually announce itself by cessation of periods and it is rare to have it appear at age 22. Suspect a hormonal issue or a lack of the trigger from the brain to ovulate.
I had really long cycles in my 20's for over a year (and out of nowhere) where I had periods every 3 to 4 months. I had some blood work done and had two issues going on. I had a slow working thyroid or hypothyroidism as well as I was naturally producing too much estrogen. For me, it righted itself while I was waiting for an appointment for an endocrinologist. Never took medication for it. I couldn't take the pill as with too much estrogen, I was having melisma and hyper pigmentation which was not making me happy (had the mask of pregnancy when not pregnant, talk about annoying! Went away not long after my hormones got back on track, thank goodness). But, many women DO take the pill to get their periods and cycles back on track with success. So, your doctor may suggest that.
Have you gained a little weight or have dry skin? I ask because those are signs of hypothyroidism too. But honestly, it was so subtle for me, like 5 pounds extra and maybe a bit more dry but I've always had dry skin. So, the biggest indicator for me was the issue of these long cycles and infrequent periods.
Bottom line, Anniebrooke is right. You need to be seen by a doctor. they can draw blood and see if your thyroid function is at the right level and what your hormone levels are and go from there as those are two very big reasons why women experience what you are experiencing. good luck