I would guess that you are having hormonal spotting. Brown color means old blood. However, sometimes, although not common, women get pregnant after a tubal. Depending on the method, either the clips come loose or if the tubes were severed, the scar tissue can form in such a way that there is still a bit of a tube that sperm can get up into. The percentage of pregnancies that do happen after tubal ligation being ectopic is very high so you should take a pregnancy test to rule out pregnant and if you are pregnant, get checked immediately. Otherwise, guessing you are having spotting based on your hormones. good luck