short cycles and early period cramps

Leah
Hi! This is only my 2nd month TTC, but by nature I'm a worrier and I've been wondering a few things. I have short cycles, 21 days usually. I always start to get period cramps about a week before my period comes. If my egg does get fertilized, does it even stand a chance at implanting if by 4dpo I'm already period cramping? :(
Thanks in advance!