painful positions

so as for the libido i have no problem. i seem to want it more actually, and i get wet just fine. so what is the problem you ask? well after almost 6 years i was getting sick of the same positions all the time so we decided to do doggy style which we hadnt done in years. i thought i was dying it hurt so bad. i literally was like oh dear god no get away from me. it felt like i was being ripped open. normal sex is fine, me on top hurts a little but not that bad and some other ones are okay but we can not do that one? anyone else have that problem? anyone know why hat position is a problem? it just makes me wonder