breastfeeding change cycle. pregnant?!

Hi, I'm really confused and wondered if anyone has been here.. I had a period on 14 feb- was full on for 4 days. Used ovulation tests and peaked on 27 feb &I also thought I had ovulation pains on the 26th. However, I went to be part of a study today and they asked me to do a pregnancy test and it was positive! Very faint tho. I can't work out how this could have happened! I am breastfeeding so thought maybe that could have altered my cycle? It was my first month off the pill