Does it have any consequense if i have sex with a girl during her period?

Not usually...: It is very unlikely that she would get pregnant but it has reportedly happened.

If ok with the blood: Some people are bothered by blood, while others are not. Sex during her period can be a bit bloody, so one consequence is blood-stained sheets, clothes, furniture, etc... Pregnancy is possible, but much less likely. Sexually transmitted diseases can happen, as usual. If she is feeling fine during her period, sex should feel fine also, without causing pain or cramping.