Abnormal Period?

Generally I have a heavy flow and a period that lasts at least 5 days. However, my latest was light and I experienced sharp, stabbing pains one night on my left side. I bled but I only had to change my pads once a day unlike 3-4 that I am used to. My period only lasted about 3 days. Any reason why this could be?

I have been sexually active for 1 year.

20 years old.