A.G.
She could have a virus called Hand, Foot, & Mouth. I just went through this last week with my son. He would yell and yell - he was in pain and we thought it was due to his mollers coming in however, when we took him to the Dr. he said it was the virus. There was nothing we could do but give him Tylenol and Motrin (what you are doing now). Im not saying this is what she has, but it is a possibility. My son is also 18 mo old.
Good Luck - I know it is so hard to see them in pain and it is hard on you as well!!