HCG trigger shot and vaginal bleeding

So I just went for my mid cycle ultrasound and I have 2 good follicles ready! They gave me an hcg trigger shot in preparation for an IUI tomorrow. About an hour after receiving the shot I went to the bathroom and I noticed a small amount of bright red blood on the paper. There were 2 small drops of blood in the toilet as well. The Internet says that bleeding and cramping can be common during the ovulation process after an hcg shot. But within 1 hour?? That seems awfully fast! I called the office and they said maybe it's trauma from the ultrasound or maybe just hormones and I'm ok to continue with iui tomorrow. It just kinda took me by surprise...I want to know where the blood came from. I asked the nurse if it could be from a follicle rupturing and she said no! What else could it be?? Has anyone else experienced this? Thanks a lot!