according to your description there is possibility for narrow cervical os, which can cause excessive pain during periods
and also less and intermittent flow,
this can occur because of any procedures performed on cervix like cone biopsy
etc, in some women congenitally also there can be narrow cervix,
in case of cervical fibroid
or polyp etc also we can see these type of symptoms,
so you better consult gynecologist an undergo physical examination and if needed ultrasound
which will possibly find out the cause and then treatment can be taken accordingly.