Catrin,
Are you moving your hands around while you ride? I started having problems at the end of last year, and then I realized I was riding with my hands more forward on the hoods than I used to. I had to tweak the nose down a hair to compensate. Of course, this didn't feel right this spring when my core was weak from the winter, but I built back into it.
"Well-behaved women seldom make history." --Laurel Thatcher Ulrich
'09 Trek WSD 2.1 with a Brooks B-68 saddle
'11 Trek WSD Madone 5.2 with Brooks B-17