Abnormal period.. can someone help?

I'm still having periods, but they're nothing like they used to be. My last two cycles have been a lot longer. I'm pretty clockwork, but these cycles lasted 38 days and then 40 days. Both times I only bled for 3 days when normally I bleed 5-6 days. Before I only got cramps the very first day of my period w/ no warning signs of it coming on. Now I've been cramping at least two weeks prior and it lasts throughout my period plus some. They're aweful and a lot of times it doesn't even feel like normal cramps. My stomach is always so upset. I'm only 21 years old so I'm not going through menopause and my life has been going great w/o much to stress about. I also haven't changed my diet making me lose or gain a lot of weight. I would just like to know if I should be concerned and see a doctor or anyone else has had an experience like this. Please help!