What do I do..long read I need advice.

Background: I had my first child 1/1/19 he will be 9months October 1st. I just moved back home after 6 months of living in Florida. I'm currently living with my mom again while my fiance and I save money to move out with our son. While I've been home I got insurance again and decided to go to the doctors about some complications I've been having and to get back on birth control. I went in to discover I have a dermoid cyst on my left ovary and was diagnosed with PCOS. I started the provera to get my period and it didnt work. So I started my metformin and scheduled an appointment for 9/17 to get my birth control in. So I go and I pee in a cup answer all their questions and my midwife comes in and closes the door. I'm thinking she's going to say something about my sugar or something or that they messed up my order and they didnt have it.

She tells me we cant put in my birth control and still dumbfounded I'm like "why" she tells me I'm pregnant. I lost it and started to ugly cry and begin telling my midwife why I cant be pregnant (like that changed the fact that I'm pregnant) they had done a pregnancy test about a month ago and it was negative so I could start my medication. So I'm dumbfounded, how I was all of a sudden pregnant when I haven't had sex in almost 3 weeks maybe more. So she draws blood because shes worried it wont be a viable pregnancy and we come up with a reason to tell my mom why I didnt get my birth control in and we scheduled an appointment for friday 9/20 to see how far along I am and if the pregnancy is still viable since they called yesterday and told me my HCG was high.

But my whole family thinks I'm going to get my birth control in. I have no clue how to break it to my mom that her already packed house with my older sister her three kids, my adopted sister, my mom her gf, my son my fiance and I. Is going to be even more full come summer. She begged me not to get pregnant and I honestly did try to prevent it but obviously it wasnt enough. I cant get excited about this pregnancy because I'm terrified of what my mom is going to say or do. I'm 21 I've been with my fiance for almost 3 years I've been out since I was 18. When I finally fell on my face and hit a hard spot in life this comes along.

What should i say to my mom? How should i tell her? Should i not tell her?