It may be recurrence of the ovarian cyst
or a new cyst in the left ovary.
There is hormonal imbalance that causes irregularity in the menstrual cycle.
Get a repeat ultrasound scan of the pelvis done to rule out this problem.
Thyroid problem in the form of Hypothyroidism
can also cause you to miss a period. Hence, get a thyroid profile done too.
You may be started on Oral contraceptive pills
to regularize the cycles.
Wish you a speedy recovery and good health.