This is definitely something that you should discuss with your electrologist. Perhaps she can change her strategy and try another program with different intensity and timing combinations. Maybe she can switch to a different probe size? Just talk to her.
Some parts of the underarm can be more sensitive than others. Seek her advice on this. My clients are not usually red after a treatment and they can tolerate a pretty long treatment session. Perhaps she was at a level higher than 4 or 5? The intensity part sounds fine. Feedback is required when you are suffering.