For most girls, it takes a few years before their periods become regular and predictable. It is not unusual for there to be changes in the length of your period and the heaviness of your flow during the first couple years that you are menstruating. The variations you are experiencing may all be a part of your body's process of developing and maturing. Over time, as your body adjusts to all the hormonal and physical changes that take place during puberty, your period will most likely become more regular. If you continue to have concerns about your period, it may be helpful to speak with your healthcare provider.
I'm going to answer this question with a question: did you ever get a haircut from the same exact stylist and then they styled your hair the same exact way they did before and then you look in the mirror and you look totally different? Well, periods are the same way. They're different all the time. Your body is affected by the normal growth and change it goes through as you mature. Some months your period flow may be worse than others. Our Health Expert, Elisabeth should have a good explanation of why that is.
I don't know exactly why. But I have totally been there. For 2 years on and off in high school, I had the worst cramps ever. I'm talking about reduced to tears and having to go home from school type stuff. For the days when it was extra bad, a hot shower and a couple of ibuprofen (make sure to follow the directions!) would solve my troubles. It may take a little while to find out what works best for you, but you'll get there. If it gets unbearable, it's time to talk to your healthcare professional.