Hello. Please don't worry. At 19 years of age, occasional cycle irregularities can happen due to stress, lifestyle changes, dietary shifts, or minor, temporary hormonal fluctuations, even when all your reports are normal.
Regarding your ultrasound, an endometrial thickness (ET) of 7mm is actually not considered abnormally thin, it is adequate, though it explains why your flow was scanty when the period finally arrived.
Since your scan shows no PCOS, and your hemoglobin (13.4) and other tests are perfectly normal, this is likely just a temporary anovulatory cycle (a cycle where an egg isn't released).
Next Steps
For now, focus on a healthy lifestyle, a balanced diet, regular physical activity, and managing stress. If the scanty and irregular periods persist for another 2-3 cycles, a short course of medication can be prescribed to help reset and regulate your cycle.
You can connect and take consultation.