Could light and brown bleeding after taking precautions indicate implantation bleeding despite negative pregnancy test?

You can use cotton undies, sometime synthetic material may cause itching.
Discharge may be regular discharge due to ovulation.

It could be due to digestive issues like gas, bloating or constipation. But contact your gynecologist if pain pesists and goes higher up to the chest, when it could be because of high blood pressure.

As per the symptoms listed, an individual is likely to be having irregular periods. This can be attributed to hormonal imbalances, stress, or even weight changes. It is important to consult a gynecologist to establish the cause and then get the right treatment.
 

Hormonal imbalances, stress, and other factors can contribute to irregular bleeding. To help regulate your menstrual cycle, focus on a balanced diet, exercise regularly, maintain a healthy weight, and manage stress.

Abdominal pain and light bleeding are a common side effect of the medicine you took. At times, the bleeding may differ from person to person, so if it is not heavy that is fine. Just rest and keep yourself hydrated. If the bleeding stops entirely or you feel severe pain then seek medical help. 

Masturbation does not cause pregnancy. Check with your gynec for your periods.

Getting late­ periods does not always signal pregnancy, so don't worry. The­ emergency contrace­ptive pill can disrupt your menstrual cycle, causing de­lays. Stress, illness, weight fluctuations - the­se factors impact periods too. If still concerne­d, take a pregnancy test at home­ for reassurance. 

It is perfectly natural for blood flow to be inconsistent during a menstrual cycle. Sometimes it may be lighter with less cramping. It’s quite common to experience a fever and body aches while menstruating – this doesn’t necessarily mean that you are pregnant. However, if you want to make sure, take a pregnancy test after your period ends. If your symptoms are worrying you, keep yourself hydrated and get plenty of rest.