What Causes Soreness And Irritation In The Vagina?

I am 3.5 weeks postpartum. Up until now I ve had no problems. Today I developed very sore inner labia. One side in particular is extremely swollen and painful with a large ball type sore on it. I have been wearing pads and panty liners since the birth because I m still dealing with some discharge. I am wondering if they are possibly irritating my vagina and have caused an infection? Your advice would be greatly appreciated as this is very uncomfortable and I d really like to get back to normal ASAP. Thank you!