What Causes Small Pea Sized Hard Knot On Leg?

i have had a dark green/blackish stool for the past week. I was on a z pack for a sinus infection and tamaflu the week before this one. I have had a lot of post nasal drip and I can taste blood somtimes. As for food, I have been eating a lot of fudge brownies. Not something I would eat a lot of normally, But I have made several batches since I have been snowed in. One more question, I have notices a small pea sized knot on my leg, down near my ankle. It feels hard and I can move it around. I was scared it might be a blood clot since I,ve been lying around a lot. I an a 45 year old female that weighs 135lbs.