Taking Plan B while on birth control?

Is it okay to take plan B while you’re on birth control? My period just ended 2 days ago and I ended up having sex without protection. I mess up with my birth control sometimes taking it at night or the next morning if I forget so I don’t know how effective that will be but I can’t afford to get pregnant right now. And with my period being 3 weeks away, The anxiety will kill me of not knowing if there is a possible chance of being pregnant 😩