It’s completely safe. Just more messy than usual.
If you’re in too much discomfort, it’s OK to take a break, or to use a smaller size than usual. However, some women find that it’s actually easier during their period, because your vaginal muscles are relaxed to help evacuate the menstrual blood more easily.
Personally, I’ve found it a mixed bag – I might be more tense or less tense depending on how much my cramps are bothering me.