There is a round ball at the entrance of my .?

i am 21 and i had a baby when i was 19.. every since it hasnt been the same down there i get these sharp pains in my all the time. i can barely insert a tampon because of this round thing. i cant get it in unless i fight with it and then its painfull. i never had problems like this untill i had a baby. sometimes i have nasty yellow stuff down there. i dont smell the same way that i did before i was pregnant. can anyone maby help me figure out what is wrong with me??? i have no insurance and going to the dr is expensive. and im too sceared to go. let me know what you think. thank you.