What is happening? Is this normal?

I am 18 years old and I have the rod in my arm, my periods have usually just been once every few months which I've been told this is normal as it varies from person to person, but over the past 2 months I have come on my period 7 times, and 3 of them were over the last two weeks, my mum reckons I might have a cyst or polyp in my ovaries because she had one a few years ago and had to have an operation to remove it. Its not so much annoying but worrying, does anyone have any answers as to what it could be please?