This is guaranteed to happen to at-least one of your clients at some point. Some people think its bruising or an allergic reaction. It is actually none of these but its good to know how to advise clients.
Bloodshot eyes occur when you're lashing and the fumes from the glue irritate your clients eyes. This can happen due to your clients eyes being slightly open. Sometimes its because your clients eyes will flicker the whole treatment. Flickering can be caused by talking, so after a brief chat encourage your client to relax.
Sometimes it can be because the eye pads have been placed too far up and are pushing the eye open slightly.
Caffeine, if your client has had a lot of tea or coffee throughout the day this can cause eyes flickering.
If blood shot eyes does happen advise your client that they will begin to calm down over a few days.