Different cycle each month. With me?

I am 20 years old I started my period when I was 11 and I have never had regular periods. I have had a child at the age of 18. She is nearly 2. I have seen my gp about this and she explained that it's fine if comes between 21-35 days. However , last month came at 28 days and month before that came at 34 days. Every month it comes at different days. Should I be worried? Have I got something wrong with me? I bleed for about a week. Some months I have grown towards the end however the last 2 periods I haven't had any brown at all. I haven't had sex since giving birth. I am overweight. I weigh 15 stone 8lbs. And I am 5ft 3inches tall. I am prone to a lot of stress as I am a single parent and the father is making things hard at the moment. My diet isn't very good either. I eat fatty foods normally as there isn't enough hours in the day with a young child and I dont do any exercise.