Postpartum period - Late?

I understand that periods postpartum have a tendency to be irregular at first. I’m about 3 months pp, I exclusively breastfeed/pump and I have been taking the pill since my 6 week appointment (I got my period back at this same time as well). At what point should I begin to worry about my period being late and take a pregnancy test? My husband and I have been careful but my first little one (whom I am so thankful for now!) wasn’t planned either and I was on birth control when he was conceived as well. How long should I wait to take a test? I don’t want to waste money on one unnecessarily because I know my period might just be irregular. But how late is normal at this point pp? 😬 Thanks!