They are interchangeable. You can say "me estoy lavando los dientes" or "estoy lavándome los dientes" and the both mean exactly the same. Those "me, te, se" are indirect objects as long as there is another direct object in the sentence. In "Me estoy lavando los dientes", "los dientes" is the direct object and "me" the indirect. If the sentence was just "Me estoy lavando", that "me" is the direct object.

why does the word "se" comes after the verb in the first example and before in the second? / Because, "se" indicates an action by/for herself Is there any difference or are they completely interchangeable? / Both sentences means the same; they are interchangeable is "se" used as direct or indirect object? Does it mean herself or to herself? / Mean by/for herself Examples: La mujer está cepillando los dientes (she is brushing -any- teeth) La mujer está cepillándoSE los dientes (she is brushing HER teeth) La mujer está cepillándoTE los dientes (she is brushing YOUR teeth) Is the same for almost all verbs in perfect present... La mujer está escribiendo (she is writing) La mujer está escribiéndose (she is writing to herself -maybe a love letter to herself jaja LOL) La mujer está escribiéndote (she is writing -maybe another letter- for you) La mujer está escribiéndoLE (she is writing to other person) La mujer está escribiéndoLES ( she is writing to others -plural-) et., etc. etc...