My last semester of undergrad was pretty bad. I had alwasy gotten As and Bs, but it was our "make-or-break year" (I was a Biochem major -somehow they save the hardest stuff till senior year!)
Plus I was working two part time jobs (stupid stupid). I ended up with Bs, one C, and one D- 😱 in P-chem.
If you are very worried about it you can do what I did - write a little supplemental letter with your application explaining why you had a bad semester. I told them that I was well aware of the emphasis they placed on grades, but that I had made a bad choice in choosing to work so much, and that I had *learned * from the experience and was dropping work hours to concentrate more on studies. If you were changing majors, etc I am sure they will understand. Everyone has an "oops" semester, it isn't a death sentence.