"My personal medical file disclosed to my partner's mum"

My story began when I phoned an ambulance due to the fact I was having severe abdominal pains. I was taken to A&E where my partner’s sister was working on reception. At this point I did not know what was wrong with me, it later came clear I was pregnant. The problem was my partner was in prison and I had an affair. Anyway it worked out I was having a miscarriage which totally distressed me. I asked the doctor not to let the receptionist know my situation as I didn’t want her to know. One year after this happening my partner’s mum decided to tell me just before my partner was due home and that I tell him about what happened or that she would. My partner has now left me and I understand that but I feel like I have had my privacy invaded which makes me angry and upset. I would like to make a formal complaint but don’t know the procedure; can you please give me some advice on my situation.