Completely lost

Ok so I’m just found out I’m pregnant with my 3rd baby. My son just turned 2 years old and my daughter is 7months old I’m currently 3weeks & 5 days (haven’t even missed my period yet) I went to the clinic and got pregnancy confirmed today. Well the past few days I’ve been having full cramps on and off throughout the days. (Yes I know it’s a normal part of pregnancy) also yesterday and today I’ve been having lower back pains on and off (it’s not painful just annoying) I never got a symptom with my son or daughter until I was 6+ weeks. I find it really weird I’m having these symptoms and my period isn’t even missed and yet. (We told about 5 people about the pregnancy already. Not all at the same time. And everyone keep saying they think ima have twins or hope it’s twins and even before I found out I was pregnant I kept having dreams about having twins and I can’t shake this feeling I might have 2 babies on board instead of 1. Why am I having these symptoms so early?? Could it be a sign it might be twins in there? I haven’t even missed my period and I’m already starting to feel miserable.