hi, in your query, its my opinion its side effect of Etonogestrel implant : i.e.
back pain and mood disturbance, an weight gain are not a serious side effect, but Serious problems usually have warning signs. Tell your health care provider immediately if,
You have bleeding, pus, or increasing
redness, or pain at insertion site.
You have a new lump in your breast.
You have no period after having a period every month.
yellowing of the skin or eyes
You have unusually heavy or prolonged bleeding from your vagina.
The implant comes out or you have concerns about its location.