The exact cause of your symptoms and whether you are pregnant or not can be confirmed only after a complete medical history with clinical examination. You should take up blood HCG levels it is a more sensitive test for detecting pregnancy. If it is negative and you still doubt you are pregnant consult a gynecologist. You may need complete evaluation in that case. Ovulation disorders, hormonal imbalances. Illness, medication, nutritional deficiencies, stress or rapid variations in weight, disorders affecting the thyroid gland all can lead to irregular menstrual cycles. Do keep me posted on how you are doing.
Take care and regards!