As I understand things (I'm not a licensed medical professional, etc., etc., etc.), there are really only two reasons you want to consider not cleansing while breastfeeding. The first is that you could wind up souring your own milk, and your child might reject your breast as a result. The second is that you run the risk of passing toxins onto the breastfeeding child which have become dislodged as a result of the cleansing.
As to pregnancy, the second issue also applies (released toxins could find their way into the womb). Besides this, "everyone knows" that Wormwood is toxic, so Wormwood , the Tapeworm killer, is always among the herbs listed as "do not use" during pregnancy.
Wormwood's toxicity is mostly a result of a worried wine industry deciding that it needed to take measures to end the popularity of absinthe over a century ago, but no one wants to take a chance that a pregnant woman is going to suddenly have a miscarriage as a result of something herbal, so this isn't something that gets tested easily. Do you want your unborn baby being the guinea pig? Probably not. Does the herbal industry want to run the risk that someone taking herbs might decide that some herbs caused their miscarriage and have to deal with that lawsuit? Not a chance.
Clarkia is probably safe to use during breastfeeding. I can't think of anything in it which is going to be harmful to a breastfeeding baby, but you need to read up on the herbs yourself and make a decision. Don't just blindly take my suggestion, because I'm offering my opinion, not advising you to use the stuff. OK?
As to pregnancy... I'd suggest holding off on getting pregnant again until you've done some thorough cleansing. Do your cleansing, and then get pregnant, and then you needn't worry about it. But be really thorough, and really do what you can to implement a less toxic lifestyle, because it will be awhile before you have the opportunity to cleanse again. Clarkia is probably safe to use during pregnancy. But no one wants to take any chances, and unless you have a really compelling reason to be the guinea pig, you shouldn't take that chance on your pregnancy, either.