When I first read your post I thought you meant your period was lasting for 33+ days! When I had my baby, periods went up the wall - only just returning to normal after one lasting 40 days, one week break, then twenty days, had coil put in, perma-period for 35days now settling down to oldish pattern. You don't say how long ago your periods went to 26 days then 18. If you're on new contraception or other medication it could be down to that. Every 18 days is a bit often to be having a period though, if you're actually menstruating for a week you'll be losing a lot of blood. I hope you feel OK, and good luck when you go to the doctor (as I presume you will or have already done)