I need advice I’m freaking out

So here’s my story. I have a very irregular period since the start sometimes I go 2-3 months without a period. I can’t even come up with a normal cycle length because they are everywhere. I recently found out I was pregnant. The doctors are basing everything on a normal 28 day cycle. According to a normal cycle I’m supposed to be 8 week 3 days however when I went in to get an ultrasound I measured at 5 weeks 6 days. Due to vaginal bleeding (Very light pink no cramps) they believe I’m going to miscarry because my sac is measuring small and I had bleeding. I tried telling them my cycle is irregular and they are just overall not listening to me. I had a miscarriage in 2018 so I know what a miscarriage looks like and the bleeding I experienced was like implantation bleeding not miscarriage. Has this happened to anyone before? My HCG level is 30,081 and she said that’s too low for 8 weeks but it’s perfect for 5-6 weeks. I’m still getting bold positive pregnancy test. The doctor literally has me scared out of my mind. Please help me...