A bad cold and/or a stuffy nose does not prevent a woman from immersing. She should blow her nose before immersing, and try to remove any crusted mucus from around and just inside her nose. She does not need to clean deep inside the nose, and anything deep inside or any dripping mucus is not considered a chatzitzah.
If the wife (or husband) is not feeling well, they do not need to have relations on mikveh night. If a woman does not feel up to immersing on her scheduled night, she may delay immersion until she feels better. Once she has completed the seven clean days, she requires no additional bedikot and should change to colored underwear.
Please feel free to get back to us with any further questions.