Answer: Fly larvae; most likely representing involvement of the tampon in the environment rather than in the patient.
As described by Florida Fan and Anon, this is supported by the presence of different larval stages and a possible pupa with respiratory structures. Pupal casings were also seen, indicating that at least 1 adult fly had hatched. There also appears to be more than 1 genera of fly present.
If this had all been in the patient (e.g. facultative or obligatory myiasis), then we would have expected the larvae to be in the same stage and of the same genus, unless there was a wound exposed to the environment which different flies were continuing to lay eggs in.
Discussion with the physician revealed that the patient did not have a wound, and that the tampon had been brought in by the patient, rather than being removed by the physician. Therefore, the most likely explanation for this case is that the tampon had been discarded in an open trash can that was exposed to flies. The patient is now likely traumatized by finding the tampon covered in fly larvae, and is understandably concerned that she is also passing larvae in her urine and vaginal secretions. The best course of action for the physician is to explain the situation and reassure her that she does not have myiasis.
Thank you to Blaine Mathison for helping me accurately describe the flies in this case!