My menstrual cycle is all messed up?

Ok, my menstrual cycle is acting all weird. I got my period at the normal time, but it was very light and only lasted for 3 days. Then about a week after it stopped, it started again. This time it was heavy for 2 days then stopped. Then 4 days later it started again and I have now been on it for 2 days, but its a moderate flow. I am just so confused. There is no way I am pregnant. However, about 3 and a 1/2 weeks ago I got a cortisone/steroid injection in my shoulder for an injury. Could the steroids be the reason for the crazy cycle? Or should I be concerned with other issues like menopause...I'm 35.