If you finger a girl like 5 days before her period, can you start her period?
Sometimes, vaginal stimulation can facilitate menstruation, but it's not a reliable method of triggering a period. periods happen and they can change in length and starting date. Girls can skip months or have two in one month. It's really just a matter of hormones, stress levels, and biology.
If you want to trigger a period, fingering a girl won't do it.
If you finger a girl and are worried about having touched period blood, don't worry.
Natural lubrications in the girl can change over the course of a month, depending on several factors. If you're worried, just be sure that you properly disinfect your hands and always wash hands before vaginal stimulation.