What Causes Irregular Periods And Weight Gain?

hey i am suruchi 29 yrs old, 5 yrs of marriage and i have a son 4 yrs old. In october i had delayed periods for 18 days, i had the medicine regestrone. then i had my periods after having it for 5 days. then in november and december my periods were on time. then again in January i got delayed periods. it was 12 days delay. this time i dint take any medicine, it came on its own. In feb also i got my periods on time. But the problem is i am gaining weight. Should i go for ultrasound or other tests are also required ???? please help!!!!