What could be causing my vaginal opening to swell out and try and force tampons out? btw i have had a clear pap smear

?Allergy?: It might be an allergy to a component of the specific tampon you are using. Could try pads or different brand of tampon next cycle and see if no swelling, then allergy. If swelling the same / similar, then should see your gyn (while having swelling / symptoms) for exam and diagnosis.

It could be...: A vaginal cyst or a forgotten tampon. See your doctor right away. Good luck!