I have two observation of the Bubble neb separated by 101 days showing a star flare up in the field of view of ngc7635.
From its position, it looks like the star in question is the known Mira type star MO Cas. It is a long period variable with a period of 252 days so the interval between your images is about right to bring it from near minimum brightness to near maximum.
I went to SIMBAD http://simbad.u-strasbg.fr/simbad/ and did a search with a 10 arcminute radius around NGC 7635 and found two mira type stars in the list that was produced. I clicked on them and plotted the field using the Aladin applet and found that MO Cas was the likely candidate.