We are sorry about your miscarriage. Some menstrual irregularity is fairly common for a few months afterwards.
Your status depends on the laws of stains. A light brown stain with no hint of a reddish tint does not make a woman niddah. Other browns may or may not, depending on the exact shade, on the surface on which they are found, and on their size. Those factors affect evaluation of your earlier stains as well. Please see our site's articles "Stains" and "Toilet Paper" for more information. Then please get back to us with any follow up questions about your status.
Even when staining continues off and on for a few days, that does not mean it is characterized as a flow. We do recommend abstaining from relations when there is staining that does not make a woman niddah (because it is less than the minimum size, etc.) but that has a niddah color. This serves as a safeguard against a flow beginning during relations, though it is not a strict halachic requirement.
If discharge is a non-niddah color, even a flow does not make a woman niddah.
Please write back with any further questions.