Spotting with twins?! (no pic)

Caitlynn
I've been having on and off brown spotting since Sunday. I'll be 12 weeks Friday. Had any mom's or mommys to be of twins experienced this and the babies were ok? I check their heart beats periodically through the day and they do have heart beats and I can hear them move around. I have appt tomorrow for ultrasound but I'm really worried. Had 2 mc before and am very scared I'm losing these rainbow babies