What Causes Anal Rash With Crhons Disease?

I was prescribed amoxicillin for a sinus infection where the post nasal drip went into my lungs, however, I developed a rash near my anus that has not gone away (though it has gotten better) after discontinuing the antibiotic. I am a 34 year old with Crohns disease that has been in remission for 10 years. I would like to know if there is anything I can take over the counter for this or is this something I should go back to the doctor for.