J.,
Yes this happened to me and I was afraid that I was preg. I didn't want to be at the time. I asked the dr and he said that it is normal to not be regular after breast feeding although I was with the others after breast feeding. He told me not to worry about it that it would eventually straighten itself out. He said what causes it was different things like your age, how many kids you have had, hormones (hormones are going to be different with different ages and times), whether I am in a relaxed environment, my body, the things that I am eating,.... he said that there were so many different things that could cause it that there would be no way that he could list it all. But the main point is that he said it could last 1 month to 2 yrs. Well it lasted about 6 or 7 months and then I straightened out. Now i am like clock work, so I guess he knew what he was talking about. If you know you are not preg for sure, I wouldn't worry too much about it. The only problem that I had is that I didn't want to talk the pill right away and without being regular, I couldn't hit my ovulating date on time and I felt nervous alot for that reason which was one of the reasons that he mentioned (stress).
I hope this helps.
M. T