My flow changes sometimes, is it normal?

skying
Usually my period comes in a heavy flow in the first 3 days, and then it lasts more 4 days, but with a low flow. But, sometimes, my period comes like only spotting, and it lasts more days. It only happens sometimes, and I don't know if I should worry about it. Is it normal???
ps: sorry about my bad english, it's not my first language