5months postpartum & so scared of pregnancy

I’m 5 months postpartum, breastfeeding every 2-4 hours just starting solids once a day. I have had one “period” since having baby- it was bleeding for 3-4 days but not clotty/ heavy like my pre pregnancy periods. Last period was Nov.16th. The last week or so i’ve had fertile mucus but negative opks (using to avoid pregnancy). I started the mini pill 2 days ago. Last night we had sex and hubby got caught up and didn’t pull out!!!! I’m terrified that i could fall pregnant. I thought about taking plan b but it’s my understanding it will not work if i am ovulating. I also find it odd that i’m having fertile cm on cycle day 30?? Any advice is greatly appreciated