Spotting on letrozole?

I have been taking letrozole since cd 3-7 at 5MG and then DR said it didnt help my follicles grow as they were stuck at 1.3 for a few days so 5 days ago he prescribed me 10MG of letrzole and today is my last day taking it. I have an ultrasound tomorrow. Ive noticed since taking it that i have darkbrown/dark red spotting when i wipe. Its only if i wipe once if i wipe again theres nothing there. This is the second day during my cycle that i have noticed it, but ive never had this happen to me before...has anyone had this happen to them before? Im super scared and anxious. Im going to ask my doc about it tomorrow but was wondering if anyone else had this issue? Please let me know!