Bleeding between periods

Hi, I'm a 22 year old virgin. I'm not on any kind of medication at all. My periods have always been irregular, ranging from 1 1/2 to 2 months between them, occasionally more but not much more. They've always been heavy too, lasting for about 12 days with about 5 days of the red bloodflow, and then becomes the dark, old blood for the rest.

Anyway, between the last few cycles (this one, and the last one) I've had a dark brown discharge. It starts out thin...like just in the normal clear discharge...it turns a light brown, it doesn't stain, and is odourless. Then gradually over 24 hours it gets thicker and heavier and starts to stain. I have to use a pad. It looks just like the old blood at the end of a period, except this time I've noticed there is a little tint of red in it. Last time this happened it lasted for about three days, and both times it's happened almost exactly 26 days after my period started. I shouldn't get my next one for another 3-4 weeks.

Does anyone know what it could be? Should I be worried? I'm not sure if I should tell my mum or not so she can take me to the doctor (I live at home and don't have a driver's license, so if I go to the doctor, she will have to know). I'm kind of scared to tell her and I don't wan't to worry her unless I have to. I think if it happens again, I will definately tell her, but in the meantime, I'd just like some opinions of what it could be?

Thanks!