Idk what to think..

I took plan b (4/24) and bled for 3 days a week later (4/2/4/4), then a week after bleeding for 3 days, i started to spot only when i wipe and that happened for 3 days (4/12-4/14). On the first day of spotting (4/12) I had cramps, but only for that day. On 4/15 I went to the ER for something else and they did an hcg blood test- it was negative. They did an ultrasound and the result was a heterogeneously echogenic material or a mass with no significant Doppler flow signal within the uterus measuring 1.1 cm in thickness. The Dr said there could be a possibility that the “material” is a sac forming but that I needed to follow up with my OBGYN. I have a follow up ultrasound next week but I’m thinking of changing it for tomorrow morning since their office is open on Saturdays. Any insight on this would be greatly appreciated.