Since you have a history of premature labor
and short labor, I presume that you are landing into premature labor.
Pains you have mentioned if they are of gradually increasing intensity and duration and regular then they are no doubt labor pains. Association of greenish discharge is serious as this can be an genital infection
which predisposes to premature labor.
Also you say you felt a bulge on self examination which might be your bag of water which might have also started leaking considering the discharge you have. This water can appear green and thick when there is infection.
Snot like mucus discharge itself indicates that the cervical mucus plug
of yours has been released. This plug seals the cervix
and is released when labor sets in due to dilatation of cervix.
You should not attempt self examination as it increases the chances of infection. An examination and evaluation by gynecologist is a must for you.
Kindly consult personally before it is too late.