Do I have to wean to get pregnant?
Probably not, but you might have to night wean if you want to keep nursing during the day. Kellymom.com states,
Current research indicates that nursing frequency and total amount of time at the breast per 24 hours are the most important factors, rather than the time of day that the suckling occurs.
The site also points out that abrupt changes bring back fertility more quickly than gradual changes. I am not suggesting you cut off your child, but rather once your child sleeps through the night by their own choice or you stop pumping during the day, your likelihood of pregnancy would increase.
NursingTwo has more information, and there are numerous resources on the bottom of the KellyMom.com page I linked. However, it is hard to find information on irregular periods in the post-partum period. I have tried, and one thing that I am doing that might be helpful is charting my temperature. I am doing it to gain an understanding of my cycles and prevent pregnancy, but you can use it to see when you are fertile. I use Fertility Friend's free service.
I hope this helps!