You just may be having an allergic reaction (which could quite possibly stop your from having a period), have an STD, or pregnancy.
Go to your doc to get a pregnancy test and see if you have any STDs. They can do it all at once from a blood draw (some blood for the preg. test and some for the STDs). Try not to scratch and touch other areas of your body, as you could spread it.
The only way to know for sure if you're pregnant is to take a test. You're already 2 days late so now is a good time to do one. As for the breaking out...you said when you scratch it seems to appear in other places. That, to me, sounds like a rash. Pregnancy CAN make certain women break out...but a break out will not spread by scratching..only rashes do that.
Take an HPT and see your doc for the rash.
not sure about the breaking out in "that area", try taking an hpt. if you are noticing 'bumps' in that area, you may also want to get checked for std's. not to scare you or anything, but just for safety.