confusing cycle

Hi everyone,
I'm a bit confused... I stopped the pill about 6 months ago. My cycles range from 28 to 40 days, so not really regular. The last one was different than usual, no known 'symptoms' and the period itself was also different (brown blood and less). For safety's sake, I took a pregnancy test, but it was negative... My period started on 11/19, and since yesterday I've suddenly had sore nipples, which I normally get during ovulation. I took an ovulation test and it seems positive, which I find quite strange. Does anyone have experience with this? Could I suddenly have a very short cycle? Thanks in advance for sharing your experiences!