It seems likely that the “little to none” menstrual periods your girlfriend experienced 2 years ago were not "real" periods. The hormonal function of the axis hypothalamus-pituitary-ovaries was probably still “sleepy”. Now, as the axis begins to “wake up” the relationship between the hormones are not yet balanced.

It seems that what is happening is that the follicles in the ovaries have been stimulated to grow and to produce estrogen hormones but ovulation is not established yet because there is no progesterone. Large amounts of estrogen hormones stimulate the endometrium to grow. However, without progesterone, estrogen stimulation can not be stopped so the endometrium grows more than normal and becomes thicker. The endometrium is the lining of the uterus that women actually bleed every month during menstruation.

So, when the time for your girlfriend's menstrual period comes, this overgrown endometrium bleeds more than normally. This condition is a kind of functional bleeding called metropathia haemoragica juvenilis. Easy cases are treated symptomatically but severe cases are treated with hormone therapy. Before diagnosing this menstrual disorder as a functional condition, all organic reasons for such bleeding should be excluded first such as : blood coagulation disorders, tumors, infections, foreign bodies, traumas etc. Your girlfriend can consult a gynecologist for these diagnostics and further advice.