It is fairly common for women with Mirena to experience staining for months after insertion, and it can be difficult to differentiate between heavy staining and a light flow.
Staining can still be considered staining even when it persists over time. A typical rule of thumb to differentiate between staining and flow is whether a woman feels she needs to use a pad. If a pantiliner would be soaked through, that would be a flow.
Based on your description, that what you have is “much lighter than a regular period”, we would treat your situation as one of staining.
You may want to discuss with your physician the possibility of taking ibuprofen or bioflavonoids, which can reduce the incidence of staining.
Please write back with any further questions.