Use condoms unless you are 100% comfortable going without them. I would recommend taking some time to learn how the pill works, how it needs to be used and what to do when you’ve not used it correctly. From the sounds of it you use it correctly. There was no point in taking plan b as plan b stops ovulation from happening, but the pill prevents ovulation, so plan b could do nothing. It’s a huge dose of hormones so you may experience side effects for weeks or longer (make sure you are aware of the potential side effects of plan b so you know what they are when they happen). Cramps are a side effect so it’s not abnormal that you’re experiencing that.
You have no reason to be concerned.