Omg so frustrating!!

Hey ladies!!! My period is 15 days late which isn’t normal for me. My period is always early or on time. I took 5 pregnancy tests and they all came back negative!!! Annoying right? If I was to be pregnant I’ll be 6 weeks so I don’t understand how my period can be this late and I’m getting back negative results. I’m not stressed or anything. My boyfriend knows some sperm went inside of my vagina last month while I was fertile and my ovulation date was either 23rd or 24th of December. Even tho I have an appointment with my the doctor in 2 days I just decided to take another test because I’m anxious and that test came back negative as well. I asked my doctor for an ultrasound when I visit to make sure I’m really not pregnant and she said I have to be pregnant to get an ultrasound. I just really don’t want to take another urine test just for it to say negative and I really would like to know if I’m pregnant or not when I leave the doctors office instead of waiting days to get a blood test result. Have any of you ever had a late late period? If so what happened? Mind you guys I have 2 kids. I’m healthy, there’s never been anything wrong with my cycle. It comes and goes normally like clock work unless I was pregnant. So this is weird to me. Also, my last period in December for the first 2 days of my period it was lighter than it normally was because usually the first 2 days are heavy for me. But even tho it was light I was on my period for all of 5 days so idk if that plays apart in this too.