A single small, dark-red clot after brushing and gargling is most often a minor gum bleed. This can happen if brushing was a bit vigorous or the gums are slightly inflamed. Dark red rather than bright red usually means it's old blood that pooled briefly, not fresh bleeding from the throat or stomach.
Next Steps
Watch for the next day or two. If it was a one-time thing with no vomiting and no blood in your stool, it's unlikely to be from the stomach or throat. A soft-bristled toothbrush and gentler brushing can help if your gums are a little sensitive.
Health Tips
Please see a doctor or dentist if this happens again, if you notice bleeding gums often, or if you ever vomit blood or see blood in your stool.