If you have taken your yasmin pills regularly without delaying or missing any pill, then there is no chance of pregnancy
Incase you havent used it prroperly then get your urine test for pregnancy.
Side effects of scanty bleeding, heavy bleeding, premenstrual spotting can occur with yasmin. For few initial cycles ,these problems occur with any hormonal contraceptive
. If its been more than 3months of use and you have the bleeding problem , then you should visit your doctor and switch to some other pill.
Also get a serum thyroid
, FSH and LH, Prolactin
tested as scanty menses can be due to altered levels of these hormones and they need to be treated.
Stress can alter cycles.
If your periods were irregular and scanty before starting yasmin, then it might be that the dose of yasmin is not sufficient for you as yasmin is a low dose pill. You havent mentioned about your age and BMI.
Scanty menses can be an indication of approaching menopause
Also these needs to be evaluated by a gynecologist for which you need to visit your doctor.