If you had faint positive pregnancy test results and the ultrasound did not detect a pregnancy in the uterus, it's possible that the pregnancy did not progress or was very early on. So the bleeding could also be due to hormonal imbalances or other factors. Check with a gynecologist to be rest assured

Menstrual cycles can sometimes vary in length and its common and nothing to worry about. However, if there is a possibility of pregnancy it's important to consider that as a potential cause. Get tested to confirm. 

After sex, it usually takes 1-2 weeks to tell if you’re pregnant but sometimes there are signs like nausea, tiredness, or changes in appetite which occur when a fertilized egg attaches itself to the wall of the womb. To make sure though, take a home pregnancy test; it’s easy and will give you an answer.

Chemotherpay drugs cause irregular or temporary stopping of periods. See your gynaecologist

It could be normal DISCHARGE or an STI.. Get tested..