What is wrong with my guinea pig?

A while ago, my guinea pig (female) had a large poo stuck in her bottom. I immediately took action and wiped it a bit then put her back when nothing happened. The same day it had gone. About a month later this happened again. As before, i wiped it. This did not help either! I have gradually noticed for the past week or two that it has cleared a bit. I bathed her yesterday (thursday 30th july) but that did not help much either! Can anyone tell me what is wrong and what i should do?