Hi Anonymous and thanks for your question.
What you have described sounds similar to the information that is provided by the company which makes Nexplanon.
According to the manufacturer, " The most common side effect of NEXPLANON is a change in your normal menstrual bleeding pattern. In studies, one out of ten women stopped using the implant because of an unfavorable change in their bleeding pattern. You may experience longer or shorter bleeding during your periods or have no bleeding at all. The time between periods may vary, and in between periods you may also have spotting."
The manufacturer also advises the side effects can include many different types of pain. " Other common side effects reported in women using NEXPLANON during clinical trials include: headaches; vaginitis (inflammation of the vagina); weight gain; acne; breast pain; viral infection such as sore throats or flu-like symptoms; stomach pain; painful periods; mood swings, nervousness, or depressed mood; back pain; nausea; dizziness; pain and pain at the site of insertion."
Generally they are saying many women will have these side effects. They advise seeking immediate medical care for the following situations: "...if you have pain in your lower leg that does not go away; severe chest pain or heaviness in the chest; sudden shortness of breath, sharp chest pain, or coughing blood; symptoms of a severe allergic reaction, such as swollen face, tongue or pharynx, trouble swallowing, or hives and trouble breathing; sudden severe headaches unlike your usual headaches; weakness or numbness in your arm, leg, or trouble speaking; sudden partial or complete blindness; yellowing of your skin or whites of your eyes, especially with fever, tiredness, loss of appetite, dark-colored urine, or light-colored bowel movements; severe pain, swelling, or tenderness in the lower stomach (abdomen); lump in your breast; problems sleeping, lack of energy, tiredness, or you feel very sad; or heavy menstrual bleeding."
If your symptoms worsen, or you experience any of the issues listed in the paragraph above, you should not wait for your appointment and be seen immediately.
Hope that helps,