How far along I am?

So I recently found out I was pregnant. I went to the ER thinking I had a UTI but came back from what the doctors said I’m 3 weeks. I don’t know how pregnant your suppose to feel since this is my first baby but I’ve been feeling really out of breath, cramping and all around sluggish. My forecast tells me I’m 4 weeks and 6 days but I feel like I could be farther along or pregnant with twins. I just wonder because I was feeling on my lower abdomen and didn’t feel the same as it usually does it felt a lot more protruding. I just need advice on what going on with my body.